I specifically use it because while I'm committed to writing free (as in freedom software), my downstream users may not be. I'd rather they didn't have more options than I do if they decide to compete against me. Free software empowers the downstream user, so I kind of think of it a bit like surfing. You potentially have all this extra energy, but that energy can crush you if you don't walk a very tight line. As the upstream author you have a bit of an advantage, but it's easy to squander it if you don't think carefully about what you are doing. I'm always a bit perplexed when people write free software but intentionally avoid copyleft. There are lots of places where permissive licenses are the right way to go, but also a lot of places where your are just creating an uneven playing field with yourself in the worst position.
I use the AGPL for user facing applications. I personally have nothing against someone taking my project and improving it as long as the project stays opensource. If they somehow manage to turn that into a business model it's fine too but what I have seen is that companies use their resource advantage to destroy the original project if it is permissively licensed.
With libraries it's obvious that you are doing it for the benefit of application developers and most of those are proprietary but they are not competing against you, they are collaborating. Therefore a permissive license for libraries is a much better choice.
Enforcing and incentivizing collaboration also seems like a good argument for licensing open source projects with the AGPL. It prevents people who are not willing to contribute things made using your software back to open source from using it.
