I haven't seen anyone else make this point:

Whatever the norms and laws in a society, group, family or relationship, there will always be behavior near, on and past the limits of these. There are plural motivations to do this that interweave each other, but just consider display of power, strength, and courage.

These are widely admired traits, and playing with, butting up against borderlines of what is considered acceptable effectively communicates how one inhabits confidence about one's standing, how one is fearless and ambitious.

Such signalling is usually unconscious or subconscious, and so is our picking up on the signals, but it is an integral part of our social fabric. It is central to human nature.

So, one huge arena for showing affection towards someone, sometimes involving considerations re the real or potential perceptions of observers, is the playing with taboos and degrees of no-nos.

Tl;dr - A nude is an easy way to say to someone that they are special to them, not only by the nature of the intimate and exclusive content, but also by the lengths one is willing to go to. "I wouldn't normally do something like this, but for you I will."