They have many uses:
To help us make the sound noises so we can say words
To sense things like food and textures
To stop things going in our mouths (we keep them closed if cycling to stop swallowing flies)
Also to signal to others, perhaps why some people wear lip gloss?
I am sure there are more of than just these
Behaviourally they allow us to generate expression, individuality and character in our faces that can convey mood/emotions/feelings. Functionally they are sensitive and allow us to detect textures, temperatures.