The commercial advantage is that it levels the playing field: If I create a library that can be used for a website and release it as FOSS under a license that is not as restrictive as the AGPL, I basically create a disadvantage to my own business: Another company could use my code in a closed service and thus benefit from my work without giving me and their users the courtesy of benefiting from their work in return.

I write commercial GPL software and will, in my next refactor, license certain parts of the software as AGPL. An example would be a library that connects an invoicing system to an online payment processor (ie. creates a full checkout process for invoice X at amount Y). I have a lot of work invested in the code (basically 5 years of my life) and the better and more reusable I make it, the easier it is to just retool it for another application. I would naturally love to see websites utilize the code, even if it is just in a SaaS setting. But since it is my work, I have decided to set the rules and if people want to use it, I think it's fair to ask that they should promote the same liberties that they received from me.