There are some sellers who feel their participation in helping facilitate a transaction should be close to zero. They are partially correct: a professional agent is hired to do most of the work, but there are certain things that only a seller knows, and when they do not share that intelligence early (preferably up front), they can trigger distrust and often cause a sale to fall apart. The sale process should be a partnership whereby the agent is the most active partner, but the seller should not disappear entirely.
We have all sorts of disclosures about properties and those are very effective most times. Inspections can also protect buyers by revealing things the seller did not know. Sometimes there are things that may not qualify fully for that disclosure that should be identified early. Whenever issues emerge from the shadows at the last minute, the immediate fear from a buyer is that this was done so purposefully. Often they are right. And sometimes, it's a case of a seller merely thinking it's 'no big deal'.
Everything in real estate is a big deal to a buyer, most of whom are in the midst of heightened emotions. Uncovering something that appears 'hidden' can result in an unnecessary volume of explaining, documentation, and hopefully regaining trust. One thing can trigger doubts about all things, slow the process down and potentially kill a deal. The costs to all are profound. Worse, the time that this eats up often has a cost associated with it.
Simply put, proactively addressing all and any issues or negatives early, up front, saves everyone time, aggravation and energy (and allows time to identify a solution).