It really is to show off. There is no reason to do it. They just like showing that they are big, and strong, and powerful. Etc.
To evidence this further, while talking to a guy that you think is into you, notice his arms and hands. If his muscles are tense, and he is stiff, then he is flexing his arms to get you to notice his strength. Also watch his hands. Guys use hands subconsciously to draw attention to parts of their bodies. If he drapes his hands in his pockets, he is drawing attention to his penis as well, showing that off.
Guys are show offs, muscle kissing is just one more way to make ourselves feel good.