Dwell is the time that the solenoid is energised, switching the flow of air to the bolt. It is the time allowed for the bolt to move forwards and stay forwards. As air is released when the bolt is open - and assuming that at any given LPR pressure, the time to move the bolt forwards remains the same - then the biggest effect is that air can flow through the bolt for longer.
Depending on how the marker is set up, this could allow you to run a lower operating pressure. Alternatively, it could just waste air.
Longer dwell will tend to improve consistency and help eliminate bolt stick.