Yes, with pressure differences it prevents oil spitting out of the front bearing. The cap is to prevent that dirt is pushed/sucked into the bearing but that is mainly an offroad issue. With onroad engines there is no need for a dust cap over the front bearing.

Normally with or w/o it should not give any tuning advantages, the crankshaft is surrounded with oil working as a seal as also the groove in the crankshaft and crank case close to the front bearing is working as a seal (and there is the small chanel for to make it work)

You can get tuning issues when the bearing gets play, then the gap at one side of the shaft is getting too large so the system of pressure and oil is not working anymore.