On occasion, people have reasons to prefer that their property not be on the “open market.”
They may wish to minimize disruption to their busy lives. They may not wish for family, neighbors or colleagues to be aware they are selling; or they may have tenants, or even vacation rental guests. Or for other reasons they may not want signs or advertising, or Open House. There may be pets or family members who need to be undisturbed. Or the property might take a long time to sell, and marketing off-MLS is a way to keep the timeline private.
Certain exclusive properties are inappropriate for the general public to view, and should be shown only to qualified buyers.
I have marketed and sold properties off-market just as successfully as when on Multiple Listing. It all depends on focus, expertise, and relationships. I have my network of agents with whom I share off-market listings.
There may even be a strategic advantage to exposing your property privately, or to only the CBBOV network of agents, ahead of MLS. There can be exclusivity in this that benefits both sides of the transaction.