Context: In cosmetics, skin toner or simply toner refers to a lotion or wash designed to cleanse the skin and shrink the appearance of pores, usually used on the face. Toners can be applied to the skin in different ways:

Question: what is toner used for on the face?
Steam of consciousness below:
The relevant information to answer the above question is: In cosmetics, skin toner or simply toner refers to a lotion or wash designed to cleanse the skin and shrink the appearance of pores, usually used on the face.
So, the answer is to cleanse the skin and shrink the appearance of pores.