An "extra man" is an
escort for rich
old ladies. There is a need for extra men because of
sociology and
biology. The husbands of
rich old ladies tend to be rich men. Rich men tend to die off well before their wives do. Good
capitalists who accumulate wealth tend to have jobs that are high stress, and therefore shave some years off their life - this leaves behind widows.
The wives, who live years longer than their husbands still have social functions to go to, and they need an escort for everything to be good and proper. That's where the extra man comes in. Extra men tend to be men of great taste, but of little means. But extra men are never paid, at least in money. As Jonathan Ames puts it in his comedic novel The Extra Man (which I highly recommend), "[I am] paid in pleasure- a free meal, good champagne, and a good orchestra. There is nothing wrong with receiving these things, especially when you can't get them otherwise'; and, 'We all inherit something, I inherited a mind. They inherited money. I inherited a certain charm... a joie de vivre, which is nice to have. I give them wit and conversation and they give me food. It's a fair exchange."
It should be noted that Ames differentiates between "extra men" and "walkers". Walkers are men in their twenties that are complete free loaders, and sometimes marry the women they escort. Extra men, by contrast, never ask for money, and never have sex with or marry the women they escort. It's simply not part of the arrangement, as the women that have extra men would never remarry, as it would sully the memory of their dead husbands.