The best use of lip liner for me is filling the whole lip before lipstick. It works like a base coat. Even if the lipstick wears off after lunch, the liner leaves some colour behind. Cheap liners can be dry, though, so I warm the pencil on the back of my hand first and use balm earlier in the routine.

I think liner is optional for sheer lipstick or tinted balm, but important for red, berry or brown shades. Those colours show every uneven edge. I use a liner close to my natural lip colour, not always one that matches the lipstick exactly. That way it still looks natural when the lipstick fades.

Lip liner makes a huge difference for me, especially with darker lipstick. Without liner, my lipstick moves into tiny lines around my mouth by the afternoon. I do not overline much; I mostly trace my natural shape and fill in the corners. It gives lipstick something to hold onto and makes the edges look cleaner.