One "useless skill" I learned long ago was the knight and bishop mate. I use it any time an opponent refuses to resign. In actuality it helps get better at my "chess vision" for knights and bishops, where my opponent would have just been better off resigning and spending that time working on their game some other way.

For the record, I usually play 20-30 games a day for the past 20-25 years, and it's only come up once without me forcing it. So it really is pretty useless compared to what else you could learn in the same amount of time.