This tool is there so you can change the target / targetnames of whatever entitiy brushes are copied (from within Trenchbroom)
It is true that if you put entities in a group in Trenchbroom, and you only have 1 targetname, you can do it from the group entitiy itself, when there are multiple targets/names, you're out of luck (in Trenchbroom).
With this tool, you could use it to copy setups with lots of different targets / targetnames. Heck, you can even use it for entire levels.
Feedback / feature requests?, Discord: @nathnolt