I'm not a model and never will be (many years and a different face too late for that) but I feel sexy when people like and comment on my stuff. I get off to people getting off to me.
I'm not here to make money, but prostitution is one of my kinks, so getting tips plays into that very heavily. I promise $5 makes no difference to a salaried professional financially - but I sure do cum harder thinking about being an (e-)whore.
tl;dr - I like the attention, it is nice to be appreciated and helps with some of my own insecurities.