...when you could use AGPL instead? Why not always use AGPL by default? What's the downside?
In my opinion GPL has been made obsolete by AGPL. I would always choose AGPL or BSD license.
The extremist GPL and AGPL should be condemned and banished from the face of the Earth. There is no reason for freedom loving people to use the oppressive GPL and AGPL. Support true freedom with public domain dedications and simple BSD-style licenses.
Say no to FOSS extremism. Say no to the GPL and AGPL.
GPL and AGPL are the FSF's long-term tools to become the largest software company the world has ever seen. The FSF is aiming to own the copyright to all software. Once it has achieved that, it will reveal its true colors by transforming into the monopolistic FSF Corporation.
Non-Affero GPL only exists to make a few corporations happy. See https://www.gnu.org/licenses/gpl-faq.html#SeparateAffero
What's the benefit of having some program code and not having its slightly more readable equivalent?
If it is about relicensing, use AGPL. If it is about new software, please don't write any new software, already more than enough software exists, we don't need more. Just stop.
If it is about new paintings, please don't write any new painting, already more than enough painting exists, we don't need more. Just stop.
I use the AGPLv3, and no later version, for most of the software I write. It just adds a paragraph to the GPLv3, and it's a better license for those who care about Free Software.