What does it look like? This dildo has a flat base, which is easy to join to a harness. Strap-on dildo is can be attached to thighs, panties or shorts, where a dildo holds at a crotch or pubic bone level. Though may be strapped to a chin or thigh, which allows more various positions. Some strap-ons are not limited in attachment to humans, but to any other object, like furniture, for instance. These are also hollow dildos, which have space for penis.  

What is the purpose? Harness dildo can help you to enjoy sex without using hands, just like your own organ.  Some men, who experience difficulties with maintaining or retaining erection can make use of penis extender dildos (hollow dildos).