Empty and for sale starts a clock
Most policies restrict cover after a stated number of consecutive vacant days, with vandalism and water damage usually first to go. A property that empties out and then sits waiting for a buyer crosses that threshold quietly, and it is precisely when nobody is there to notice a problem.
Showings mean strangers with access
Agents, buyers and inspectors coming and going is a different exposure from a tenanted home. It is manageable and it is worth the carrier knowing, because an injury during a showing is a liability claim against the owner.
A tenanted sale keeps the landlord policy running
Where the tenant stays through the sale, the landlord policy continues and transfers at closing in the usual way. The complication is only where the property empties, which is also the arrangement sellers usually prefer.