There are actually a number of different reasons why you get a particular package.
Some packages are just basic,and you expect to find them. Much of the GNU stack comes in this way. And often things like Perl and Python are a foundational requirement for a lot of other tools.
What I want personally
There are a number of areas where I have specific interests - I'm a bit of a magpie when it comes to X11 window managers, for example. And I need to open office documents, so I had to get LibreOffice working. There a few games or emulators that I like. This also explains why some things might not be present too - I have no real interest in video or multimedia, for example, so that's an area with relatively little coverage.
Oh, that looks cool
I'm often interested in new stuff. (Even if it's actually old stuff that's just new to me.) So if I come across a piece of software and think "that looks cool" I'll often try and build and package it. If it works, fine, it ends up in the repository. Even if I might not end up doing anything with it, I've gone to the effort of making a package so it may as well stay there and somebody else might make use of it.
I have a $DAYJOB
Yes, I have a day job (a very good one, thank you very much), and it involves running applications on illumos. If I'm going to evaluate software I'll do it on Tribblix first. Building stuff on Tribblix is much easier than on, say, OmniOS - I have total control of the environment, and many more tools and prerequisite packages to give me a head start. So I can easily screen out any software that simply isn't going to work, and identify any patches or modifications necessary, before heading into the rather more constrained work environment.
Can you make X or Y or Z available
I get requests from users. I'll pretty much always at least try to add the software asked for - the fact that someone's bothered to ask indicates it might be useful, and I might find it interesting as well. This doesn't always work, of course, and I'll have to punt.
I got bored one day
Sometimes I get a bit of free time (no, this doesn't happen very often), and start looking for packages that might be worth adding. Sometimes this involves looking at other systems to see what they ship.
It's a prerequisite for something else
Dependency hell is a fact of life, so a lot of the time is actually spent building prerequisites. This is one reason for speculatively trying things out - it identifies prerequisites, and they're often going to be needed by other packages too. Although what you'll find is a number of packages with no obvious consumers, because the software I wanted didn't actually work. As I mentioned before, though, I'll keep those packages I built, and they might come in useful later.