Because you consume Google content but you don't give as much surveillance data to Google as you would with the official YouTube client spyware, you skip advertisement which denies Google revenue, you can do things like listen to music in the background which are only available for pay in the official client - again denying Google revenue...

And of course you give Google the finger. Spite is also a valid reason.

It's not much but it's something, and at this point anything that hurts Google delays the inevitable dystopia a little bit - and sadly I'm afraid it's a little too late to hope for much more now.