If you're a lady, you want to be discreet. Many men will come and go--sometimes at the same time, but sometimes you have that one dude that you can always rely on. He might live with you, although he might not be your husband. He might float you a little cash occasionally. He's got your back, and you don't want to dissappoint him. He's usually a lover.
A person says "main man," especially so others may hear, to insinuate that they have a number of friends; the target of "main man" is just one of many friends that this person has. When a person resorts to saying "main man" outloud, he or she wants the world to know that he/she has many friends other than the target of "main man," so often the person who says "main man" may have insecurities or personality traits that result in him/her not having many friends.
The homeless person, who does not have many friends, said "main man" to me to suggest that I was just one of his many friends.