The things AGPL adds to GPL don't just affect people trying to do a SaaS offering of the program. If you modify it and users interact with it over a computer network you have to make source for your modified version available to them.
For example suppose it was software to add online ordering to restaurants. A restaurant modifies its copy so that it can be given the recipes of the items they sell and the modified software uses that information to allow customers to easily exclude items they might be allergic to or that violate their religious or ethical eating rules.
If that restaurant wants to use that as a competitive advantage over other restaurants they aren't going to want to have to give away their modifications, so aren't going to want to use AGPL software. They'd probably be fine with GPL software.