To give kisses.
The lips are part of our sense organs. These as a whole detect our environment and allow us to alter our behaviour accordingly. An example would be my ears detecting a train coming and me getting off the track. Our lips contain many temperature sensors. If we consume food that is too hot, we may burn our tongue or mouth or throat, then those areas my swell up and prevent us breathing efficiently. Our lips prevent us from eating food that is potentially too hot.