I actually use it for a variety of reasons, but still not often. I'll list them here in order of what I believe to be most useful to least.
1. To throw off the opponent and keep the unexpectancy level up.
2. To block other projectiles.
3. To use as a ledge move; holding ledge, drop down, air jump, tap B, grab ledge again.
4. To throw off opponents using Popgun Cancel. Cancelling into a grab works well against some people, because since peanut has the second lowest knokback in the game, they'll come in to attack, and you grab em.
5. To dodge attacks or to recover. These are together because they are executed the same. In the air charge it and let go, and it will push you back. This can be very useful for mindgames, throwing off the opponent, and if facing the right way, recovery.
6.To break so you can eat the peanuts. It's hard to do this, but it's always useful just to pull out the gun for some healing oppurtunity.
And lucky number seven, To do damage. Lol, okay but really, it's pretty underrated, and people should see some uses that it has. Not often does the right situation arise, but when it does, have the Peanut Popgun in mind. Diddy for god tier! :D