Because most believe that swapping = bad and that if you don't reduce swappiness, the system will swap when it really doesn't need to. Neither of those are really true. People associate swapping with times where their system is getting bogged down - however, it's mostly swapping because the system is getting bogged down, not the other way around. When the system swaps, it will have already factored the performance cost in to its decision to swap, and decided that not doing so would have a greater overall penalty in system performance or stability which may later become noticeable.
Overall the default settings result in good overall performance and stability. I'd recommend leaving it at the default. There are improvements that could be made to how swap works on desktop/interactive systems, but by and large the swappiness control isn't the best fix - adjust it in one direction and you may fix one issue and create other issues. It pales in comparison to simply installing more RAM.
How Linux uses RAM
Any RAM that isn't being used by applications may be used as "cache". Cache is important for a fast, smooth running system, speeding up both reads and writes to disk.
If your applications increase their memory use to the point they are using almost all your RAM your cache will shrink and on average disk operations will slow down as a result.
If applications increase their memory use even further - assuming you have no swap space - you will eventually run out of memory and your system will have to kill processes. Killing processes is worse than a slow down as it results in instability.
How Linux uses swap
To combat both of these problems, your system can re-allocate some seldom-used application memory to the swap space on your disk, freeing more RAM. This can prevent processes dying due to running out of memory, and can reclaim a little cache so disk operations can operate smoothly.
This re-allocation isn't done according to a definite cutoff though. You don't reach a certain percentage of allocation then Linux starts swapping. It has a "fuzzy" algorithm. It takes a lot of things into account, which can best be described by "how much pressure is there for memory allocation". If there is a lot of "pressure" to allocate new memory, then it will increase the chances some will be swapped to make more room. If there is less "pressure" then it will decrease these chances.
Your system has a "swappiness" setting which helps you tweak how this "pressure" is calculated. It's normally not recommended to alter this at all. Swapping is overall a very good thing - any occasional performance penalties are intended to be offset by a gain in overall system responsiveness and stability for a wide range of tasks. If you reduce the swappiness, you let the amount of cache memory shrink a little bit more than it would otherwise, even when it may really be useful. You therefore risk slowing down your computer overall. If you go so far as to disable swapping completely, then you risk processes being killed due to not being able to allocate memory.
What is happening when the system is bogged down and swapping heavily?
Swapping is a slow and costly operation, so the system avoids it unless it calculates that the trade-off in cache performance will make up for it overall, or if it's necessary to avoid killing processes.
A lot of the time people will look at their system that is thrashing the disk heavily and using a lot of swap space and blame swapping for it. That's the wrong approach to take. If swapping ever reaches this extreme, it means that swapping is your system's attempt to deal with low memory problems, not the cause of the problem.
What about desktop systems? Don't they require a different approach?
Users of a desktop system do indeed expect the system to "feel responsive" in response to user-initiated actions such as opening an application, which is the type of action that can sometimes trigger a swap due to the increase in memory required.
One way to try and tweak this is to reduce the swappiness parameter which can increase the system's tolerance to applications using up memory and running low on cache space.
However, to an extent this is just shifting goalposts. The application may now load without a swap operation, but it will leave less slack for the next application that loads. The same swapping may just occur later, when you next open an application instead. In the meantime, the system performance is just a little lower overall due to the reduced cache size. Thus, any benefit from the reduced swappiness setting may be hard to measure, reducing swapping delay at some times but causing other slow performance at other times. Reducing swappiness a little may be justified if you know what you're doing, but reducing it to as low as 10% should be done with caution, as this can leave the system tolerant to very low cache sizes and leave the system more liable to have to swap at short notice.
Disabling swap completely should be avoided as you lose the added protection against out-of-memory conditions which can cause processes to crash or be killed.
The most effective remedy by far is to install more RAM if you can afford it.
Can swap be disabled on a system that has lots of RAM anyway?
If you have far more RAM than you're likely to need for applications, then you'll rarely need swap. Disabling swap probably won't make a difference the majority of the time. But if you have plenty of RAM, leaving swap enabled also won't have any penalty because the system doesn't swap when it doesn't need to.
The only situations in which it would make a difference would be in the unlikely situation the system finds itself running out of memory and consequently the cache system is getting hampered, and it's in this type of situation where you would want swap most. So you can safely leave swap on its normal settings for added peace of mind without it having a negative effect when you have plenty of memory.
But how can swap speed up my system? Doesn't swapping slow things down?
The act of transferring data from RAM to swap is a slow operation, but it's only taken when the kernel is pretty sure the overall benefit as a result of keeping a reasonable cache size will outweigh this.
Once data is in swap, when does it come out again?
Any given part of memory will come back out of swap as soon as it's used - read from or written to. However, typically the memory that is swapped is memory that has not been accessed in a long time and is not expected to be needed soon.
Transferring data out of swap is about as time-consuming as putting it in there. Your kernel won't remove data from it if it doesn't need to. While data is in swap and not being used, it leaves more memory for other things that are being used, and more system cache.