The advantage is timing, not leverage. A buyer who sees a home a few days before it hits the open market gets first look and first offer, which matters in a tight market at 1.63 months of supply, but it does not mean the seller is motivated to accept less than market value.
Where off-market access genuinely helps is avoiding a bidding war on a listing that would otherwise attract multiple offers the day it goes live. Getting in before that happens can mean negotiating with one interested buyer instead of five.
It is a real advantage worth having, just not the shortcut to a below-market price that it is sometimes sold as. Get your hand-reviewed valuation from Joel Dyck.