Because it feels terrific.
Lubricant is a vital part of vaginal and anal sex.
Lots of lubricant is a must with latex. Vaginal lubrication doesn't automatically follow sexual arousal and doesn't necessarily indicate sexual arousal. Lubrication is influence by hormonal fluctuations, many women experience dryness especially if they have had a hysterectomy, have reduced estrogen levels, or are menopausal. FTMs who enjoy vaginal penetration will likely need and enjoy lube, if they are taking testosterone or if they have had hysterectomies. Women and FTMs will also find that clitoral stimulation is more pleasurable with lube.