The engine can only produce its rated power if it is allowed to turn at the speed required for that power. If full power is at 2700 RPM, then restricting it to 2400 RPM will limit it to something less than full power.
Most light aircraft engines are not rated for continuous full power output; continuous power (>5-15 minutes, depending on the engine) may be restricted to 65-75%. Hence the reduction to 25 in/2400 RPM after gear & flaps are retracted, and at some safe altitude.
First, check your owner's manual for any restrictions. Then, within those restrictions, realize that for a given power output, an engine works most efficiently at wide open throttle (WOT) and a speed commensurate with the available MAP. So, if you have a choice of running at 24 in/2400 RPM or 25 in/2300 RPM or 26/2200, the latter is better. Rumors of engine damage due to "oversquare" MAP/RPM combinations apply ONLY to supercharged engines, and are not generally applicable to light piston engines.
Note also that engine overhaul limits/recommendations are based on "tach time," which is more a count of engine revolutions than actual clock time. So, running at lower RPM -- still within all limits -- will reduce maintenance costs somewhat.