Why? Because it’s good for you.
It’s good for you to think outside yourself.
It’s good for you to give something, even if it isn’t what you’re good at. Especially if it isn’t what you’re good at.
The economics of specialization are still in play. It’s more efficient for you to spend the weekend doing freelance work and use that money to pay movers to help your friend.
But it’s good for you to be inefficient for somebody else’s sake.
Because you can’t optimize friendship. And inefficiency says “I love you.”