Noise control, not banging the props off the fine pitch stops, enough excess performance at 2400rpm to not need full rpm, pilot discretion, etc? If you can find a copy of the engine manual then you should find a table showing what MAP can be used with what RPM, ie what the highest MAP you can safely run with each RPM setting is. Makes a nonsense of people setting 25/25 in the climb in something NA because "it's max continuous power".