The short answer: ESSENTIAL!
The long answer: Women perceive foreplay as part of sex itself. This means that sex starts at kissing, hugging and touching erogenous zone, not penetration.
Foreplay builds trust, encourages relaxation and builds the sexual tension.
If you want great sex, make sure your woman BEGS to you for penetration into her. Make sure she goes really emotional and agitated.
Also, it is a great idea to wait one minute to stick the dildo or your dick into her. Touch the outer area or the clit first.
Formula: More begging leads to higher quality sex.