Your skin's oil is slightly acidic. This means it has a slightly low pH. As it gets distributed onto the surface of your skin, the acidic nature of your skin's oil helps to kill bacteria that falls on it. That's why oil accumulates around your eyes, nose and mouth, aka the T-Zone - the oil gets solid by mixing with dead skin cells, so that some oil should stay there on a permanent basis and protect these orifices. Once the oil and dead skin cell mixture builds up enough, it hits the hair getting exposed to oxygen, which makes this accumulation turn dark brown, appearing as blackheads. Here's the kicker - if your skin's pH gets thrown off and isn't acidic anymore, it will over-produce oil in order to get its status back to acidic, which can make for more blackheads. If the pores that contain these blackheads get inflamed for any reason, that's how you get a breakout. All breakouts begin with microscopic blackheads.
Now, if you take a base, or alkaline, substance (something with a higher pH) and mix it with fat, what do you get ? Soap! Most cleansers have a higher pH than the skin because most cleansers are alkaline in order to degrease the skin and lift dirt away. Some are the same pH as the skin in order to be more gentle, but don't cleanse all that well. Others have a very high pH and get the skin squeaky clean, but those strip the skin like crazy and dry it out. The best cleansers clean the skin thoroughly while keeping the skin from getting dry. Either way, the possibility that the skin's pH is going to get thrown off by a little, or by a lot, is fairly high.
That's where toner comes in. The best toner is slightly acidic and therefore balances the skin so it doesn't have to re-balance itself by producing more oil. Blackheads, and therefore breakouts, are kept more under control. It's very important to remember, the skin does not overproduce oil in order to restore moisture - notice that when you use a cleanser or soap that dries your skin out and apply moisturizer right after, your skin will still be oily afterward! This is because your skin is trying to balance its pH, not its moisture.
Toner also adds water to the skin which is a double bonus because the more hydrated the skin is, the more pliable it is, making skin soft enough for the oil not to get quite so solid and stick around for that long. The best moisturizers have ingredients that attract water molecules to themselves and therefore bind water to the skin, which means if you apply it to skin that is damp from a balancing and nutrient-rich toner, you end up with much better results from your entire skin care routine.
So, with toner you get skin balancing and hydration at the same time, making skin softer, producing less blackheads, and giving hydrating moisturizers plenty of water to hold onto the skin.
Why Toner ?
Remember your high school biology? Remember all about acids and bases? Litmus tests, pH of 7 being neutral? Skin has a great connection with this concept. Your skin's oil is slightly acidic. This means it has a slightly low pH. As it gets distributed onto the surface of your skin, the acidic nature of your skin's oil helps to kill bacteria that falls on it.