Detonation is for real. Detonation is still not well understood. People have tried all sorts of things to make it happen but it never happens reliably.As long as the bullet gets out of the barrel who do I care if the load is below the minimum? I am thinking of plinking, not defense or hunting, and not where power factor matters. I understand that higher pressure means more complete burning, higher spin rate, better consistency, and so on. I can't think of a safety reason, as long as the bullet clears the barrel.
Detonation is when the rate of combustion exceeds the speed of sound in the medium. Under that point it's deflagration. Pressures can rise to the point where barrels zip apart, breeches break open.