Usually, when you slacken off a tripod head, the camera will move freely and easily – often too freely and easily for precise positioning. So some tripod heads have a friction control to offer some resistance, even when the head is slackened off.
This stops the camera flopping unexpectedly and possibly being damaged. It also allows more careful camera positioning by hand.
Friction controls are most often found on ball heads, where precise positioning is often quite difficult. They may also be found on video heads, where it’s more likely to be called a ‘drag’ control.