As a multi disciplined martial artist, it’s kind of cool to be able to change up routines if I’m hurt or someone else is hurt. If someone has hurt their knees, we can focus on our boxing, or our punching as an example. If someone hurt their ribs from a ripping body shot, or perhaps we can do some pad work and not use any heavy contact. You can do striking, grappling, make it a more technical session rather than intense. 

Same can be done with anything. I know a guy who does triathlons. He said he can run, and when he can’t run, he can jump on the bike or in the water.