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. It's true that there are certain times when swapping can have a noticeable penalty, but reducing swappiness for that case can reduce overall system performance or stability in other ways which may later become noticeable. Overall the default settings should result in the best overall performance and reliability. I'd recommend leaving it at the default, or at least between say 40 and 60.
I hereby reproduce the answer I've given to another question here on AskUbuntu:
How Linux uses RAM (very simplified)
Each application can use some of your memory. Linux uses all otherwise unoccupied memory (except for the last few Mb) as "cache". This includes the page cache, inode caches, etc. This is a good thing - it helps speed things up heaps. Both writing to disk and reading from disk can be sped up immensely by cache.
Ideally, you have enough memory for all your applications, and you still have several hundred Mb left for cache. In this situation, as long as your applications don't increase their memory use and the system isn't putting too much pressure on the cache, there is no need for any swap.
So, when someone refers to "free" RAM, this may or may not include cache, since cache will only occupy "free" RAM. The cache will shrink as RAM is occupied by more critical stuff, like applications.
How Linux uses swap (even more simplified)
Once you have used up enough memory that there is not enough left for a smooth-running cache, Linux may re-allocate some unused application memory from RAM to swap.
It doesn't do this according to a definite cutoff though. It's not like you reach a certain percentage of allocation then Linux starts swapping. It has a rather "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, and I would certainly never recommend you alter it. Swapping is overall a very good thing - although there are a few edge cases where it harms performance, if you look at overall system performance it's a net benefit 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 in general, because there is less cache, while memory is being taken up by applications that aren't even using it. Whether this is a good enough trade-off for whatever specific problem you're having with swapping is up to you. You should just know that reducing swappiness can slow down the system for tasks that make heavy use of the system cache, that's all - and that's IF you encounter a situation where you even need swap.
There is a well-known situation in which swap really harms perceived performance on a desktop system, and that's in how quickly applications can respond to user input again after being left idle for a long time and having background processes heavy in IO (such as an overnight backup) run. This is a very visible sluggishness, but not enough to justify turning off swap all together and very hard to prevent in any operating system. Turn off swap and this initial sluggishness after the backup/virus scan may not happen, but the system may run a little bit slower all day long as there is more pressure on the cache size. This is not a situation that's limited to Linux, either.
When choosing what is to be swapped to disk, the system tries to pick memory that is not actually being used - read to or written from. It has a pretty simple algorithm for calculating this that chooses well most of the time.
If you have a system where you have a huge amount of RAM (at time of writing, 8GB is a huge amount for a typical Linux distro), then you will very rarely ever hit a situation where swap is needed at all. You may even try turning swap off. I never recommend doing that, but only because you never know when more RAM may save you from some application crashing. But if you know you're not going to need it, you can do it.
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 will outweigh this. For example, if your application memory has risen to the point that you have almost no cache left and your I/O is very inefficient because of this, you can actually get a lot more speed out of your system by freeing up some memory, even after the initial expense of swapping data in order to free it up.
It's also a last resort should your applications actually request more memory than you actually have. In this case, swapping is necessary to prevent an out-of-memory situation which will often result in an application crashing or having to be forcibly killed. There's no getting around the fact that your system will probably become dog-slow when this happens, but console yourself by realising that were it not for swap, your application would likely have crashed instead, potentially resulting in data loss.
Swapping is only associated with times where your system is performing poorly because it happens at times when you are running out of usable RAM, which would have slowed your system down (maybe even crashed applications) even if you didn't have swap. So to simplify things, swapping happens because your system is becoming bogged down, rather than the other way around - and in some cases it can save the day.
Once data is in swap, when does it come out again?
Transferring data out of swap is (for traditional hard disks, at least) just as time-consuming as putting it in there. So understandably, your kernel will be just as reluctant to remove data from swap, especially if it's not actually being used (ie read from or written to). If you have data in swap and it's not being used, then it's actually a good thing that it remains in swap, since it leaves more memory for other things that are being used, potentially speeding up your system.