[mindslight]

I don't think the question is so much asking "why", but rather "what" does "access to my own ... reserves at a moment's notice" even mean in a debt-based financial system?

Writing a check is instantaneous, but you obviously don't mean that.

If you're talking about wires and the like, then what you're really paying for is the sending institution taking on liability if there turns out to be a cascading problem.