My yoga mat is one of the most useful things I own. It allows me to easily practice yoga, which helps me control things that can't be willed away, like muscles that prefer tension and a spine that often feels like stone.
Yet, it's still a luxury. Yoga is meant for everyone, even those who can't afford rectangle-shaped rubber. The extra padding and stability justify its cost, but a few cardboard boxes and a couple towels work too.
My yoga mat is useful to me. But I know one day it might not be around. My knowledge of yoga will be.
The things inside our head are worth more than the things outside.