Why a lean-to?

Okay, this will sound super-naive, but my wife and I have done quite a bit of hiking, biking and outdoors-ing in the ADK. We are going to branch out a bit and try some kayak-camping and bikepacking. As an alternative to tenting, we may decide to use one of the many lean-tos in the park. So my questions are these: what is the benefit of a lean-to and I’ve always wondered why they don’t just enclose them. Is it for the view and campfire warmth? Seems odd to me to build 3/4 of an enclosed shelter and just stop haha. No bug or bear protection. Am I looking at it wrong?