A financing condition is the clearest example. If a lender needs one more document and the buyer does not respond for several days, the condition date can pass before approval comes through, and at that point the seller is entitled to walk away from the contract entirely.
The same logic applies to a seller. A slow response to a home inspection request for repairs or a price adjustment can push a buyer, especially one with other options in the current market, to simply move on to the next listing rather than wait.
In a deal built on deadlines, being reachable is worth as much as being reasonable. Get your hand-reviewed valuation from Joel Dyck.