Capt.Nemo's answer is great. On a more general note, consider this:
An operating system, such as Ubuntu, consists of thousands (if not millions) of lines of code, all written by human beings. Every piece of software you run, on top of Ubuntu (Firefox, Thunderbird/Evolution, Gnome-Shell, Pidgin, etc), range anywhere between a couple hundred to several thousand lines of code, written also by human beings (usually different ones).
Human beings are fallible creatures, and maintaining that much code gets confusing very quickly, even under the best of circumstances. It can be easy to forget the side effects something has, change something marginally related, and introduce a bug, because of the interaction between the two marginally related things. It can be even easier to write something that you think is good (or, in some cases, just something that works, but is ugly, due to a time crunch), and have to go back and redo it, because it introduced a bug, or worse, a security hole.
To further add to the difficulty, there are hundreds of different computer setups, which can affect the functionality of a given piece of software (particularly at the OS and hardware driver levels, where things aren't as abstracted). Sometimes, no matter how much testing a team does, bugs don't even appear until the software is released into the wild.
Therefore, the updates you get every day, or every week, are for those little changes, bug fixes, security fixes, and various other little updates. If you use third party PPAs to get the latest versions of things, then it might even be larger version updates, which include new features.
The most common logical mistake people do when it comes to software is compare it to physical structures, such as bridges. Bridges can be declared "finished" and not need touched for many years (save maybe for repairing the roadway part, where cars drive and wear it down). Bridge design can be tested and retested in theoretical environments (from using math to determine the optimal angles for strength, to scale models) and the requirements for the bridge can be determined at the very beginning of the project. None of this works for software development (as repeated failure of Waterfall projects has proven time and time again), where there are times when the developers may not even know that a given requirement exists until production time, thanks to all of the little details that come with software development. Therefore, a software project is never considered "done," that's why there are versions and releases, and feature additions to begin with. Those daily/weekly/monthly updates are the product of constantly updating and improving and tweaking of a piece of software.
Or, to put it another way, if everyone waited to release their software until there were no more bugs, at all, and nothing needed tweaked or fixed, and the software was absolutely perfect, no one would release any software. (That's not to say everyone releases software without testing it, there are simply thresholds of what is "acceptable" for a given release, and what can be put off until post-release updates.)
That said, you don't have to update every day, or every week, or ever, really. The developers can't force the updates onto your computer. However, it's in your best interest to update periodically. Allowing security updates to install silently is a good way to make sure you get the most crucial updates, and running updates at least a couple times a month ensures that you're not fighting bugs that have already been fixed.