It keeps your code bona fide open source, so people who want to use and extend it noncommercially can do so without worrying about you at all; the code is genuinely free.
At the same time, it means that people who would attempt to commercialize your code must at least do so on a level playing field with you. They can't take your code, improve it, and withhold those improvements.
AGPL is much better than a "noncommercial" license, because code that can't be used commercially isn't open. "Noncommercial" licenses are deceptive; they suggest that you have an incentive to improve other people's code, but really you can only benefit the owners. And it's better than the BSD license, which fails in exactly the opposite direction.
People who are reflexively anti-GPL tend not to know much about the security software industry, where intense VC funding and rafts of "me-too" products have plagued the major open source projects (Snort, Nessus, etc). If your code is ever going to really be worth something, and you MIT/BSD it, expect someone else to make a lot more money off it than you do.