Reverse

Realize it's gonna be a lot of boat talk but since I'm living with it right now, you just have to deal with it. Today I learned that it is not ok to reverse with any speed with a boat. Apparently not good for the engine. It's only to be used for "corrections" as they call it. To adjust the location, to back out slowly or similar. On my (tiny) engine it says do not put reverse in when you are speeding forward (like I can with my boat?!?) which is fine. I can understand that. But why not if you start from standstill? I have so many questions and so few answers...