Th eadvantage is at the open. If you put in a bid at a certain price you go next in line to be filled at that price. If you enter a bid at market you are next in line at the open price whatever it is. A perfect example of this was yesterday am. The market  for spq showed a huge bid at 11.5c. I put in a bid at market and was filled at twelve but 30 seconds later it was at .14. Hope this helps. sometimes I find it cofnusing myself.

K