If you are playing single-player or multiplayer and pick up an overshield, during the time when it warps the shield up, you are temporarily invincible. I picked an over shield up and launched my warthog, trying to ride in it, thinking that i might survive the blast with a super shield. but it turns out it did absolutely no damage. This could be useful for executing rocket jumping / grenade jumping tricks in multi, when you want to get somewhere high without hurting yourself at all. For instance, in Battle Creek, you could pick up the over shield, then quickly rocket the ground underneath you, catapulting you up onto the enemy base allowing you to go for the flag with a full shield.
Or, you can ride your warthog when you do a warthog jump.