Besides the obvious reasons, that is.

The way in which [director Rian Johnson] decided to create the Force connection by just simply doing vertical cuts without using any CG … it’s pure simplicity in terms of filmmaking with visual cuts. We cut to her side; we cut to Kylo Ren; we cut to her; and back and forth. That was important to establish what she was actually seeing. Was she hearing his voice or seeing his face or just his eyes? And so that [shirtless scene] is to inform the audience, “Oh, she can see his body.”

TL;DR: Kylo was shirtless so that we, the audience, could understand just how much of him Rey was seeing during their Force FaceTime.

UPN

So there you go. It wasn’t ALL just eye candy.

Paramount