Perhaps if I play this more I'll understand, but doesn't the map editor let you edit maps? Why would I do this in-game?

There's a map editor?

I use it because I like my factory to look nice and instead of building on top of ore, or waiting around to mine it out, I erase it. It also removes oil spots that you no longer use because they are dried up. I also play with Bob's mods which adds a ton of ores, and resource spawn overhaul which means I ignore the close to start ores and go far afield for it which leaves a lot of small ore spots I don't want existing in my base area.

You can also use it to cheat a bit.. er I mean simplify your life a bit where you have two veins too close together with diagonal lines making it a bit more complicated to mine them without mixing them. Simply erase a nodule here or there. This is especially useful with Bob's large area miners.