If your subdividing land, or don't know where the pins are, then you need a survey. If you have a survey of your land, and need to place a shed on it, you need a site plan. I've done several site plans for my land. once to build my house, once to add a garage, once to build a barn. It won't be perfect because it's like 1"=40' or something like that. they just want to make sure your not encrouching on setbacks, septic area, etc.....
When I was building my house the excavator took out a corner pin. cost me $300 for the surveyor to come out and set a new one.
I'm working on a project now where I need a site plan. I have a copy of the survey, which I've entered in my CAD, but I can't physically find any of the pins. If I could find one, I could probably find the rest, but most of the area where the pins should be is grown up. Anyway, they might have to get a surveyor in to locate some corners, but I will still do the site plan myself.