While hand jobs or masturbation do not run much risk of pregnancy because there is no penetration, there is always some risk when coming into contact with sexual fluids of another person, especially when these fluids are around the vaginal area. Therefore, some couples choose to use a condom when engaging in play that involves sexual fluids.
The movement of the hand on the penis mimics the in-and-out of sexual intercourse, which is what makes a hand job pleasurable for the guy. This is the same motion that is mimicked by masturbation, dry humping and oral sex.
