Supposed idea is for the weld tool to be more easily usable without having to fiddle as much with all the settings I guess.
Personally I find automerge annoying to use for the simple reason that after going through the menu(in 2.79 and previous, now it’s in a popover, kind of a menu too) and enabling it, once I’m done using it half the time I forget it on and then run into issues becaused of it later. The good thing about the tools system is that once you are done with the tool you are guarantied to switch away from it and can safely forget about it. Thats just elegant.