Account binding ties an NFC pass to the member's own account, so the pass in the wallet is a personal credential rather than a transferable card. A bound pass cannot be shared: it works from the wallet it was issued to, and the wallet does not offer the pass for passing along.
The problem it solves is as old as membership cards. A plastic card or an RFID fob authenticates whoever is holding it, which means memberships get lent out: to a friend for gym access, to a family member for member pricing, to whoever the discount is worth something to. The program pays for members it never enrolled.
Account binding closes that gap at the credential level, and it is a property of NFC passes. The bound pass belongs to the member's account, so presenting it means the enrolled member's own device is at the reader, and the encrypted tap of Apple VAS or Google Smart Tap carries a payload no screenshot can reproduce. Together they make the pass a credential that is both convenient to carry and genuinely hard to lend.
Where it matters most is wherever the pass is worth borrowing: gym and club membership, member pricing, staff benefits. For a simple points card, sharing is a minor leak; for a membership, account binding is the difference between a member and a rumor of one. Not every pass wants it, either: coupons and gift cards are often meant to move between people, so binding is a choice made per program, on the passes where the credential is personal.