If you are a ultra trail runner there are a lot more trail 50ks than trail marathons. And 50ks are ideal tuneups for longer ultras because you can in time-on--feet and work on fueling strategy but they don't require as much recovery time as longer distances.
When I was running ultras I would always try to do a few 50ks early in the season as prep for a 100 later in the year.