Category vs. Queue for technician assignment? Round Robin?

I see that the Category and Queue capabilities include the assignment of a technician(s) to be members of the respective Categories/Queue's.

Is there any kind of conflict that occurs with a new request that comes in and is assigned a default category, but you want to put it into a queue instead? Or vice/versa?

I guess I'm trying to understand what the benefits are one way or the other as you can create Categories or Queue's with whatever names you want.

Additionally, the documents don't say anything (unless I missed it) about "round robin" dissemination of tickets. Does the placement of tickets into Queue's or Categories just leave it in there, or do they get assigned to technicians based on their active tickets (or lack thereof?) Is it incumbent on the Techs to pull tickets into their personal queues?