Then if you had a map with a lot of imported data (music and models) and you didn't remove or import anything, saving the map would give the algorithms a much smaller input size.
So whenever you work on a map and you don't plan on importing files any time soon, you'll spend much less time waiting for the .SC2MAP to save and be opened in the game.
This would make testing a much more pleasurable experience when being able to make tiny modifications to the map and having the ability to test those minor modifications much more quickly.
It's what I like about making small maps, they save faster and they debug quicker so modifications are almost live. It's a step closer to playing and editing simultaneously.