Players often time use procedural fuel tanks to create walls for crewed craft, texture mixes or certain buses for probes, which often uses a lot of part and reduces performance. Creating hatches for some curving wall is pain as you resize each fuel tanks in a section of wall to make holes for hatches and windows.
Having the ability to merge parts to create single part hexagonal tubes or some curvy pipes, and the ability to separate parts by creating loop cuts (or even better, just making some cuts to make a hole on any surface) for creating doors and windows for custom crew pods.
Basically:
Merging: less parts calculated for physics, more performance
Separating: makes cutting holes in inlets or merged fuel tanks easier