I've wondered this too, and I'm sure we're both talking about the really ridiculous ones, like jibberish letters and numbers registered in sequence. It really makes no sense sometimes.
My theory is they were C&C servers for botnets or something else nefarious. They appear to have been registered in bulk and are basically random.
For example, there used to be a website called McDumpals that was for selling stolen credit cards. They would assign each user of their website a random domain with which to access the hidden TOR/onion service. I've never seen it in person of course but Krebs On Security wrote about it.
Otherwise I'm like you - totally stumped.