You mean two buttons? You need to two buttons to switch maps
If you mean why switch maps at all: to be able to experiment, mostly. e.g. having different throttle tables available that I've set up a few days before the track weekend. Eventually I may play with maps for race fuels and/or maps for different rev limits (if I beef up the engine, not the least part of which would be the oil pump).
Also, I use one of my maps as a 3mph security map so I can leave the car idling somewhere and it can't drive off faster than I can walk.