The main function of a toner is to sooth your skin.  Toners it should be noted do not actually tone or tighten your skin, they can reduce the size of your pores but only momentarily.  

Toners do help return PH balance to your skin and good quality toners can help to reduce skin inflamation, return moisturising agents and provide some anti-oxidant protection to your skin and remove any last traces of make-up.