Do athletes wear those blck smuges under thier eyes for a practicle reason or just to look silly?

It's used to prevent glare.

they are motley crue fans 

They do it for the same reason the Egyptians used to wear heavy eye liner, to reduce the glare of the sun. 

I used it when I played baseball and softball back in the day. Burning a cork was just as good as the store bought "eye black." I wore it because I thought it was cool, IMHO it didn't help glare that much. There is very little defense an outfielder has for a ball directly in the sun, creating a good angle beats eye black by miles. 

You get sweaty jumping around like that. and with the sun shining the sunlight can get reflected into your eyes, blinding you.

They do that when they're hung over from the night before.  That way when they're on the sidelines and they want to take a little quickie nap, nobody can tell they have their eyes closed.

Because they cannot wear sunglasses on the field, silly.