I could use the keyboard and the mouse while scratching my head all at once.

Get half as terrible at juggling.

Two chicks at once, man.

Use the second set of arms to restrain the first set from sexually harassing me

Solder without alligator clips

Well I would only assume that I would be a much better rock climber.

FOUR ARMS so obviosly start doing ben 10 shits

Will the first thing would be to Google how to date 2 women at once.

I can finally constantly hold an umbrella

Now I truly never need to make a second trip back to the car for groceries again.

Cosplay Ryōmen Sukuna.

oh man I would have to start practicing sleight of hand again I'd be the coolest magician

I can quit my job and become a mega cashier at a grocery store. I'd be unstoppable.

Learn to play the Antagonistic Undecagonstring from The Hydrogen Sonata by Iain M Banks.