Why DO men send unsolicited pics of their penis?

Another poster asked this question in their thread; and I am curious...WHY DO men send those pics? and What do they get out of it? To be quite truthful with you, I have absolutely NO IDEA what men get from doing this. 

Guys: have you ever sent a woman, or do you KNOW SOMEONE who has sent a woman, unsolicited pics of your/their penis? WHY did you/they do it? Are they trying to attract the woman? Are they trying to turn her on? Are they just hoping she will send similar pics of herself? What do they feel when the woman reacts negatively? Do they think it's funny?