Why keep ants?
Ants are very cool.
But anyways, the gist of why you would keep ants is that they're interesting, low-maintenance, and (usually) very easy.
I personally keep over a thousand colonies, and most of them started from a single queen. The way most ants start colonies is that a single queen, without any help or even food, raises a brood of workers on her own. From there, the workers begin taking over tasks, and as the number of workers grow, you see more food come in, more workers being hatched, eggs being laid, etc. Really, it's like watching a mini society grow. Some species of ants have multiple casts of workers for different roles, others have special relationships with plants or other insects they maintain.
What separates ants from a lot of other things is that there are a lot. Scientists estimate that there are one million billion ants in the world.
So why is this? If you've ever learned anything about the environment, you probably know about food chains.
Let's revisit those. Remember producers? The plants? Ants collect nectar and sugar from plants, and some species collect seeds. Ants are primary consumers.
What else might eat plants? Maybe an insect, like a fly.
Ants eat those too.
What might eat a fly? A spider, perhaps?
Ants eat those too.
Maybe a bird eats the spider, and guess who's waiting when that bird reaches the end of it's life?
The ants.
Ants are able to effectively take in food from every single level of the ecosystem (just like us), which is why they're so successful. It's also why a lot of them are so easy to keep. Most ants need very little space. In fact, they prefer a little bit of space. For example, Carpenter ant queens build only a single chamber, usually only 30% larger than herself to start the colony. Ants also, as established, can eat a lot of things, our feeding guide covers this a bit more in detail, but it means that you won't need any special gear or whatnot to keep them.
Ants are also very clean. They have to be if they're to keep away infection in those tight spaces. In fact, most ants will create a pile of all your garbage FOR YOU. In the past, I've even gotten some colonies to move all their trash into a designated container. Lastly, ants can be kept for a LONG time. Some species, like Pogonomyrmex occidentalis, can last up to 45 years.
Ant colonies, strangely, seem to have different personalities too. For instance, a lot of Myrmica are typically very skittish in the wild. Alternatively, I've had aggressive Myrmica queens trying to attack anything they say.
Ant-keeping is also (sadly) pretty understudied. For many of the species, you might even be able to document first-time-observed behavior!
tl;dr you should keep ants