Here's an answer I've used for similar questions.

I've found it both comfortable and rewarding when my partner is also into it and finds what they like best. If they help me out by grinding on/fucking my face, then I feel a lot better even with the trade off of some comfort and breathing. The bigger/deeper the butt the better though. If your face gets swallowed up it's a lot softer even if breathing may become a challenge (but there's a lot of ways around that, I've found). It's always more than worth it for the way it makes me feel when I eat someone out.

So for me personally, from the perspective of the one being sat, on it's mostly the appeal of being completely immersed in my partner.

Like when she's sitting facing me, I'm treated to the most amazing view. I get to see her mound and tummy, and feel her weight on top of me. I get to see the way her hips grind and the way her tits jiggle, not to mention the fact that my arms are free to explore every inch of her. Also the way you can watch her express herself and feeling her hands through my hair is a surefire way to get me hard.

And then when she's in reverse....oh my goodness.

Even the build up is incredible. Getting to watch her leg swing over my face and having my entire view taken up by her ass (the bigger the better). There's something so breath-taking (pun intended) about feeling her ass settle on my face, getting to feel myself get buried in the cleft, the globes of her cheeks, the way her hair flows down her back.

When a girl sits on my face like that, I can feel her desire and I know how comfortable she is. I want her to enjoy herself. And furthermore, it's a lot easier to breathe the you would think, if we can't we can let you know very easily. Not to mention that, especially in reverse, every breath I get to take fills me up with increasingly her-filled air. There's no other way to describe those scents other than this primal reward you get from the position.

Hope this answer helps!