In the map screen you are drawing your route, that's why you still need to drive afterwards. You can draw a quick route to quickly get to the building or you can wander around to catch some ghosts with your vacuum.

I can't really say that Ghostbusters for the Master System was a GOOD game lol. I agree though, that map twisted my brain trying to understand the perspective the buildings were drawn in. I also only played the game with a money cheat so I could afford stuff. I think I beat the game a few times, the part at the end of having to time running past the Marshmallow Man sucked.