Photographers pay models 'cos they enjoy photographing models or because they want something particular for their portfolio. Or occasionally to submit something for stock.
Clients pay photographers for a job - who may need to employ a model for that job.
Established models rarely pay photographers for portfolio work. There are a lot of very talented photographers eager to work with experienced models for trade.
New models sometimes pay for portfolio work, but only 'cos they don't know where to find those talented photographers - and in fact there is a whole industry dedicated to ripping off wannabees.
PurplePort is the best UK model / photographer networking site but has rules around age limits.
Things get a bit different when agencies get involved.