If an engine has run for many an rpm, it will change the insides, most notably the top of the cylinders, for a ring of 'carbon' will build up. This ring reduces the length of the piston's travel. To compensate for that loss several things must happen, the rods must torque a bit, or the bearings must give a bit, or the crankshaft or the piston or some of all of these must compensate for a shorter available stroke.
Eventually, especially when the engine must pull a heavy load, or climb a long hill, or is suddenly called upon to run at significantly higher rpms, or is run hot or on less oil something will give.
Thus the reason to pull the head on an unfamiliar engine and check if there is a ring and remove whatever is there for with that removed, the engine now is able to run it's rpms with full freedom. By assuring that the piston has full range of motion the engine's life is extended.
In like manner the valves, cleaned and seated provides the air flow required thus a cooler temperature and thus less fatigue. Now these improvements do not a new engine make, but they may provide the amount of extended time one needs until a complete build is required or desired.
I find the basics of the mechanical arena as interesting as the infinite details, important as they may be and by knowing and improving the basics I can enjoy this hobby without spending lots of money.
Now if I can refrain from burning down the shop, I may be able to play a bit longer.