Lip liner, while it is not a necessity every time you put on lipstick, can enhance a look. Tracing a line around the outer edge of your lips with a liner pencil can prevent lip color from bleeding to give lips a polished, professional look. Since it essentially keeps the lipstick in place, lip liner can help color last longer.
When you're just starting out with a liner, it can be a good idea to choose a nude color that matches the natural color of your lip. This will also allow for more versatility because it can be used with every color.
If you want a bolder look, choose a liner that is the same color as your lipstick or lip gloss, but remember to bring the makeup with you for touchups so the liner does not stand out against your lips if the color fades. You should also avoid lining your lips with a shade that is darker than the lip color, as that trend went out of fashion in the 90s.
Using a lip liner can also be the perfect way to mask any flaws in your natural lip line, but it is best to stick as close as possible to your lips' shape.