Some yeasts can be difficult to clear with common fining methods, some ale yeasts, many lager yeasts. Less of an issue for homebrewers who have the time for a proper layering period which usually produces bright beers, for the pros, who need to turn around a bright beer in much shorter time, fining before filtering is important.
I've read some issue keisosol, others use biofine or similar products, sometimesat higher than normal dosages and still combine with a close to freezing large ring period of a few days before filtering and packaging.