In the visual arts, commercial art galleries are the primary interface between artists and potential buyers.  Galleries discover good artists and their work, then make the public aware of it, via marketing activities of varying degrees of sophistication, as well as by simply displaying it on their premises.

There’s too little space in commercial galleries for all the available work by every capable artist to be made continuously available to the public.  To exhibit significant amounts of their work, even the best-known artists have to settle for shows lasting maybe a month or so out of the year.  The rest of the time, their galleries can’t typically provide space for more than a few of their works at a time.  

This shortage of gallery space can make it as hard for painters and sculptors to get their work evaluated by gallery curators as it is for musicians to get their demos listened to by the major record labels.  It also enables galleries to impose harsh rules on their artists, such as not allowing them to exhibit at any other gallery within, say, an hour’s drive, on pain of being kicked out of their current gallery.