Why is the Recovery volume's version of Disk Utility more powerful than that on the main volume?
Disk Utility is the same between the regular OS and maOS Recovery, but the environment it's running in is different, and that affects what it can do. The biggest constraint is that Disk Utility can't do much to things (e.g. volumes) that're in use. My guess is that some program had a file open on the volume, so it couldn't eject the volume, and therefore couldn't repair or erase it. In my experience, the background process that scans and indexes documents for Spotlight searches tends to be a culprit here.

In Recovery mode, almost nothing is running (including no Spotlight indexer!), so Disk Utility is much less likely to run into this sort of conflict.

BTW, there are some things that Disk Utility can do when running from the regular OS but not in Recovery. One I've run into is resizing the startup volume (if it's MacOS Extended format -- not sure about APFS). The reason for this is that the Recovery HD partition is right after the startup volume, so it needs to be moved if the startup volume is resized. But you can't move it if you're running from it!