When you tighten the nut the collet will first grip the shank preventing bit movement. Then as you continue to tighten the nut it forces the tool “up” with the nut and collet. If the tool is bottomed out this ends up either cutting into the back of the router/spindle or more commonly keeping the collet from properly seating causing runout. In worse cases it can damage the bit, collet, or nut. So it’s best practice to leave a gap to allow the tool to move up while tightening the collet.