There are certain situations in which renting directly from an owner may not be your best option. This may be the case particularly if:
You have very specific requirements, such as wheelchair accessibility, an industrial kitchen, or three apartments in the same complex.
You're traveling with a big group. The variables—do you need one mansion? several small homes? a dozen condos?—may be too difficult to coordinate.
You don't trust the vacation owners in a region. A rental agency will be the one who's liable if anything should happen. And you can almost always use a credit card.
You can't find anything. An agency might offer you a new option to consider.