For one, the best running tights provide a wider range of movement, compared to jeans, obviously. You wouldn't want to feel restrained during running or exercising, and thanks to the stretch fabric used in most compression tights, you may even improve your stride and running cadence.
Running compression tights can also reduce muscle soreness since they keep your muscles warm and tight. This can potentially reduce injuries, too, although that said, you should still do your warm-up set in the gym and/or stretch before your runs to minimise the risk of injury, regardless of whether you are wearing running tights or not.
Wearing compression garments can also help you get through high-performance training and races easier, reducing what's called a 'perceived load'. Wearing running tights might make you feel more energised during and less exhausted after your sessions. They also promote blood flow and therefore increase the amount of oxygen your muscles get during workouts, further reducing muscle fatigue. Wearing compression tights can also improve strength recovery.