Why get a compression test?
Since the power an engine delivers is dependent on its compression and since compression can reveal how worn an engine is and help avoid catastrophic engine failure by letting you know how well valves, rings, etc are sealing, this might be a worthwhile test to reveal the condition and wear of the engine.
A compression test is especially recommended on older boats and on outboards. Why outboards? Because they are stressed far more than inboards and often run at much higher RPM. The life of an outboard is generally 800 hours as compared to 1,000 hours on a gasoline inboard and 8,000 hours on a diesel inboard. These are rough estimates and 1,000 hours at WOT is not the same as 1,000 at idle. So use makes a difference as does maintenance, etc.