These ad videos are also one of the many ways to earn free ore. Go to your loadout screen, then hit "forge" and then "upgrade" and you can use ore to power up your weapons and armor, both equipped and unequipped. You'll have more power ups available depending on the rarity of the piece of equipment in question.

You can also watch a free video to add boosts before a round begins. Normally you will not really have to do this, but when you come to an especially tough round, watch videos and see what you'll end up with. Some of the boosts will increase your exp, while others will power up your knight throughout the entire battle.