Every slow cooker I've ever used/seen would be hard pressed to hold more than a gallon or two, and can only bring it to a simmer at the most. You would never get a good boil going.
Never use a slow cooker for beer before. I think slow cooker is not suitable for bear as it may destroy the taste. If you have a slow cooker, it's better to make different kinds of soups.
This is about the only thing I could think it could be used for. But at that point, you're probably doing smaller batches so you'd be better off just putting the pot in the oven and not futz with trying dial in a temp on a slow cooker.