When you're playing or recording, you'll probably be working at a variety of tempos, and hopefully time signatures too.
Practicing - depends on your goals. If your goal is to increase your speed with something you're already doing, there's not much point in going slower - unless you're making mistakes. Then you've got to put it in reverse for a bit
If you're practicing to try and perfect something, precision is more important than speed. Precision first - speed later.
When you're learning something, you'll just naturally have to go slower.
Sometimes it's fun just to chill out and play something you know well slowly, just as a form of meditation or really feeling and perfecting a groove.
