1. It helps remove any residue left on the skin after cleansing.
2. It has a relatively low pH, helping to adjust the pH of our skin's acid mantle after cleansing. When our skin's pH is balanced, it provide our skin a good barrier function against the environment.
3. It rehydrates our skin and prepares it for moisturizers.
When choosing a toner, try to avoid toners containing SD, ethyl or isopropyl alcohol, these are very drying and irritating ingredients. Alcohol will dry up the existing surface oil and cause the oil glands to produce more oil to compensate.
The best way of applying toner is to spray it on, if your toner doesn't come with a spray bottle, you can always get one from a dollar store. Otherwise, you can just pad it on with your hands.