oapilot
A SID is a clearance.......comply with the clearance until you receive an instruction to do something else.
Question for you........if the controller had read out the clearance item by item including the step-climb limits, would you still pose the question?
A SID is a convenient way to keep R/T loading to a minimum, and may take into consideration such factors as noise route/terrain/separation from other routes/traffic etc. etc. A controller can [on occassion] issue a different clearance spoken in full, or vary one or more conditions on a published SID, should the circumstances in any particular traffic scenario deem it the right thing to do.
Rgds.