>>132883807There's certainly an aesthetic point to them. I think it's a mixture of being a point in the body where muscle definition is always readily apparent and never excessive. If you notice, most of the images, especially lewd fanart, of armpits usually portray them as a very specific degree of tonedness. The fact that they seem to show off that subtle muscle definition is certainly an important part.
Then there's the fact that it's sort of a meeting point between the arms and the chest, and naturally leads the eyes down along the contours of the body towards the hips, maintaining an impression of the overall form and shape all the while. It seems to immediately suggest and draw attention to all of the attractive aspects of the female body, especially those we associate with femininity intuitively. The typical armpit exposure pose, with the arms lifted overhead and the back stretched tends to draw emphasis to the shape of the body as well.
There's also the fact that it's a very soft, sensitive and private bit of flesh. Normally hidden, rarely exposed and usually associated with some kind of personal shame and decency. Exposing them is hot in the same way that exposing the collarbones or nape of the neck is hot. It implies all of the immodesty and intimacy that frontal nudity would, but without the direct sexuality of nudity. It's the same sort of idea as 'leaving it to the imagination'. Being indirect about intimacy can often be more enticing that being overt.
And, of course, armpitservice in anime is always in its most idealized form. Pits shaved so cleanly there's not even a suggestion of stubble. No smell, no filth, no uncleanliness. Always the suggestion of it being an area where air passes freely and not the stuffy asscrack of the upper body that it is. It's much easier to appreciate the intimacy of something gross being carelessly (or deliberately) exposed when it's exposed in a form that's anything but gross.
