It may be intended to rub against the genitals to cause sexual arousal. It can also be used to ensure that a butt plug or dildo cannot be ejected. (The plug may be attached to the strap; see harness plug.) On a male, the crotch strap may be split at the front to avoid the penis and testicles, or it may pass over them so as to squash them.
Crotch straps are often fitted to straitjackets to make them more escape-proof.