The mechanics are the medium through which players relinquish control of their roleplay. They're sort of what mediates roleplaying, I think, by providing a sort of arbitrary and neutral interface to both limit and move along characters.

I can roleplay all day and use real world knowledge to back up how my character can pick a lock, but if there's a mechanic that made me roll dice and I lost, I have to roleplay why that character just couldn't get the lock picked despite it all.

My main point though was more an allusion to game mechanics not needing to be overly specific or defined by manuals. But then again I like cooking up rulesets and customizing campaigns. Or rather I did in the Ye Olde times when I socialized.