Box and papers: what a full set is really worth
The kit in the safe can be worth more than the strap on your wrist. Reference by reference, here is what completeness adds at resale.
Modern watches: 5–15%
On current-production pieces a full set adds five to fifteen percent and — more importantly — speed. Complete watches sell in days; watch-only examples wait for the right buyer.
Vintage: the multiplier
On vintage, original papers can double a price. Punched papers matching the case serial turn a nice watch into a collection piece, because they are the one part nobody can source later.
Keep everything
Hang tags, bezel protectors, receipts, even the plastic — completeness compounds. Store papers flat, out of sunlight, away from the watch itself for security.