What's the point of telling your Companion to sleep?
Solution 1:
I can think of one possibility: If you were a vampire and needed to feed. It's easier to have your meal follow you around…
Solution 2:
You can use the skull of corruption on them. It's very useful, since using the skull on someone while sleeping increases its attack power by 50. You can use that in middle of a cave by finding something the companion could sleep on.
Solution 3:
I don't think they put it in the game like some say. From a programmers point of view I can tell you it's most likely just not turned off. I haven't experimented much with command mode on my followers, but can't you just interact with all objects that you as a player otherwise could? If so, that means it's just default. Also the reason why companions can't produce anything on tanning racks etc is because command mode doesn't include the menu choices. Hope this answer wasn't too boring, but it's probably just as boring as this :-)