The Mother of All Bombs or MOAB killstreak reward

Short answer

It increases damage indirectly.

Longer answer

This has been a common occurrence since Modern Warfare I. The 'impact' weapon perk is similar to the 'deep impact' in the first one. While it doesn't actually increase the damage directly, there are a couple important scenarios to note here that could cause your weapon to do more damage.

The first:

Chances are you're shooting through surfaces more then you realize and the increased penetration is actually making your gun stronger. Most Call of Duty fights take place around some sort of cover (unless they are a total noob and roam the middle of the street avoiding potential cover) and while you're carefully aiming at their head or chest chances are you're going to miss and utilize your bullet penetration.

The Second:

Aah, the multi-kill. My favorite reason to throw on 'impact'. In Call of Duty, players are considered a 'surface', meaning that if two players were standing in a line and you shot the first one, the penetrating bullets would hit the second player for less damage then the first. Putting on 'Impact' can be the difference between clearing a room of four players and only killing the first.

People care a lot about their stats, whether its accuracy, kill/death, win/loss, or highest kill streak. In the previous Call of Duties people often struggled with trying a new gun and getting good at it vs using a gun your already good with and helping your kill/death.

By allowing the ability to reset, people can take the game a little less seriously. They can experiment with guns and play styles they previously would never try knowing they can always reset and try again. In the end allowing players reset their stats it relieves a lot of stress from players over concern about their stats, and lets them start over from scratch enjoying that initial "unlock excitement" that a fresh account gives.