Power transmission often requires rotating shafts with sprockets, brakes, pulleys, gears, wheels, and many other types of devices attached in unique configurations.
A round steel shaft with a keyway allows for similarly keyed interchangeable components to be positioned securely along it's entire length. Such a shaft can be manfactured from round stock with as little as a broach and a hammer.
ISO 500-3 for PTO splines
ISO 5673-1 is a good start for other splines.
DIN 5480 & ISO 4156