* Swelling: Lip piercings swell a lot initially. A stud (usually a longer one) allows room for this swelling without putting pressure on the piercing. A ring would be too constricting and could cause problems.
* Healing: A stud is less likely to move around than a ring, which minimizes irritation and helps the piercing heal properly.
* Hygiene: It's easier to clean a stud than a ring in the early stages of healing.
So, while it might be tempting to get a ring right away, it's best to follow your piercer's advice and start with a stud. Once your piercing is fully healed, you can switch to a ring if you'd like.