honestly, a toner is a must for everyone, but that toner must be for your skin type. after you wash your face, any products you use and water included, increases the pH of you skin (which is normally 4.5-5 on the scale) to at a minimum of 7 on the scale (water alone raises your pH to 7)....so you need to bring the pH of your skin back down immediately before you moisturize or you can cause your sebaceous (oil) glands to overproduce oil in reaction to the unbalanced pH.
If your skin is already dry, then you need a moisturizing toner to help even out your moisture balance because washing will strip it and then your moisturizer won't penetrate well at all because your pores have shrunk because of the high pH.
So that's why, like Chelsea said, your moisturizer will penetrate better....and you won't need much at all.
For acne prone skin, an astringent may be necessary...but that's not necessarily the same thing as a toner...just depends on ingredients honestly.