The map contains information telling the ECU what fuelling and what ignition timing to add in various cirumstances.
On a 172/182 one of the things the map tells it to do is hold the fuelling at lambda one until 5Krpm or so, this is very bad for performance at full throttle under that rpm, a remap stops it doing this and allows it to go richer to get the correct mixture for peak power.
It also introduces slightly more ignition timing to increase power a little as well, although the gains from that side of things are much smaller than from the fuelling correction.