Hull and waterline
Freshwater boats pick up a waterline stain and a film that ordinary washing does not remove. Getting it off before layup matters more than getting it off before a weekend, because six months under a cover is what sets it into the gelcoat.
Topsides, vinyl and canvas
Vinyl seating cracks when it dries out and stains when it does not get cleaned. Canvas and covers need to go away clean and dry or they grow something over the winter. Both are cheaper to maintain than to replace.
Protection, not just cleaning
Cleaning removes what is on the surface. A wax or sealant is what stops the next six months of ultraviolet light chalking the gelcoat. Doing one without the other is half a job.
What six months under a cover does
A cover keeps snow and rain off, and it also keeps air still. Anything left on the hull or the upholstery in October has six months of damp, dark stillness to work with, which is why a boat that goes away dirty does not come out the way it went in.
The waterline film that freshwater leaves is the usual example. Removed in October it comes off with cleaning. Left until April it has had a winter to bond with the gelcoat, and what was a cleaning job is now a compounding job. Vinyl behaves the same way: cleaned and conditioned it survives; left with sunscreen, spilled drinks and lake water on it, it stains and then cracks.
Canvas and covers need the same treatment for a different reason. Stored damp and dirty they grow mildew, which is far harder to remove than to prevent — and a cover that has failed is protecting nothing underneath it.