My theory is one of two reasons. And, to be clear, this is not justification:

-to show you how attractive they find you. You send me picture, i use it to get hard, i send you picture of my hard dick so you can see that you got me hard. You're flattered, of course.

-to find out if youre "wasting" his time. He's out here looking for a hookup or a girl who's going to match his level of sexual energy, not a pen pal. Sometimes a girl will just chat with you, chat with you, accept your compliments gracefully, and then let it peter out. He wants to know now if youre intending at any point to touch his boner. Maybe it doesnt compute with him that you havent made up your mind about that part yet. Guys make up their minds on that pretty early.