Unless I’m working on a client, I never use lip liner. On myself, I concealer around the lips and that acts as a barrier for my lipsticks.
On my clients, I use lip liner to improve the longevity of lipsticks. I line around the lips, fill them in with the liner, add a layer of lipstick, a layer of translucent powder, and another coat of lipstick. This formula makes lipstick last all day and then some.

Is lip liner an absolute must have in your personal kit? I don’t think so! How about you?