It allows you to play in a relaxed manner and improves your rhythmical precision. I also find that I have more endurance when I take my time because being relaxed helps me to take more air which sends more oxygen in my blood and helps me playing a bit longer.
Practice with metronome to make sure you don't rush. We don't always notice that we rush when we're used to play in some way. Sometimes we make our life harder than it should be!