Depending on the cleanser, the skin might lose some of its beneficial elements in this process. This is where the toner comes in and helps restore the balance of the skin so it doesn't feel too dry or too oily.
The amazing thing about facial toners is they are water-based hence it helps bring the hydration back into the skin after the cleansing process. Some brands include even more ingredients to maximize that.
Another great thing about toners is that they come in different forms. Some are just a liquid, some are wipes and some are sprays. Spray toners are great because you can just use them so easily even on the go.
There are many brands out there making toners with various ingredients, but regardless another great function of toners is that they calm the skin and alleviate redness from cleansing if any.
I would say this is probably the most important function of facial toners to essentially end the cleansing process. It helps get rid of excess dirt and impurities that are left on the skin.