This is a great question! It is actually VERY important to at least have a monthly facial treatment. There are a number of reasons for this.
A good, active facial will help you in the following ways:
1) It will offer you a deeper exfoliation than you can accomplish at home, this will encourage skin cell turn over and allow your home care products to penetrate better.
2) It causes stimulation of the deeper layers of your skin, thereby encouraging the skin to increase in cellular activity and result in more collagen production (which may not be apparent when you are younger, but this counts for a great deal when you reach your forties and older).
3) It puts you in the hands of a skin care professional who is then able to advise you regarding anything they may detect on your skin during your treatment, as a good therapist will be assessing your skin the entire time while they are busy working.
4) Of course there are other benefits specific to the TYPE of facial you are having, for example, if you are being treated for pigmentation, the product that the therapist uses on you would certainly be more concentrated than what you would use at home, giving you better results.