To me, level drain can extend the shelf life of a character. If the game ends when you reach a maximum limit, level draining foes moves the end back such that you can enjoy your character for a longer time, especially when there are party members that level at uneven rates. You don’t want to see that magnificent character ride off into the sunset too soon. This can also be a good pacing device if you need the party to stay around the same level for a bit of time. It’s a good DM tool in moderation, but balancing the good and bad things you throw at players. Plus, most energy drain abilities are touch-based, so they can be played around with control spells and ranged weapons which can provide some relief to the hack and slash.