Why sell through a stock agency?
If you have money, time, business know-how, and discipline, then you might consider selling your photos through your own website or a portal (portfolio) service.
If you don’t have time or interest in marketing yourself and handling customer sales and service because you need to do a lot of shooting and retouching your images, then you will probably consider a stock agency.
A stock agency represents and sells the work of many photographers through their website and distribution channels, and charges the photographer a commission for their services. Since you don’t have to maintain a full fledged website, market yourself intensively, and handle sales, working with a stock agency can give you a lot of time to do what you do best; be a photographer!