Offhand shots certainly aren't necessary for hunting. But they do expand your opportunities when still-hunting.
You should be able to play to your strengths and adapt your methods to the quarry and to your weapon. Treestands, groundblinds, pitblinds, blowdowns, stumps, a tree seat. All these allow a relatively solid rest in most situations.
Another absolute must-have in your bag of tricks should be a good kneeling shot. Assuming you're right handed: left foot flat on ground, left elbow on left knee, right knee on ground, right butt-cheek on heel of right foot. It's a little more "balled-up" than a traditional kneeling position, and a little more stable. I drop into this from my ground-level tree seat if the game is behind me or to shoot around cover, or from a standing position to get under cover. Good snow-shoeing position, too. Works even better with a tree up against either hip or shoulder. I shoot 50 yard groups this way when I'm just out shooting for general practice. Very steady three-point stance (toe-knee-foot).