What a winder does
A watch winder is a small box with a motorised cradle that rotates the watch slowly to mimic wrist motion. The rotor inside the watch swings and winds the mainspring; the watch stays running indefinitely. Winders typically rotate at 650-1800 turns per day (TPD) in alternating clockwise/counter-clockwise cycles, mimicking the activity of a worn wrist.
Why most don't need one
Modern automatics are designed to start and stop normally. The mainspring, gear train, escapement, and rotor handle starts/stops without wear. Letting a watch run down for a day or two does no damage. Most watches without complications take 15-30 seconds to set on putting them on (date, time). For a daily-worn collection of 2-3 watches in rotation, a winder is solving a non-problem.
When a winder is genuinely useful
Perpetual calendars: setting a perpetual after it stops is genuinely a hassle (multiple correctors in a specific sequence; one wrong push can desync for months). Keeping a perpetual on a winder avoids this. Annual calendars: same logic, milder. Watches in collection rotation: if you wear a watch only every 30+ days, a winder keeps it ready (but does add wear over years). For most other cases, let the watch stop.
What it costs in wear
A rotor and the automatic-winding gears (reverse wheels, ratchet mechanisms) are designed to wear from regular use. Continuous winding ages these parts faster than wear-then-rest cycles. Watchmakers report rotor-bearing replacement on long-term winder watches at 7-10 years vs 12-15 years on rotation watches. The cost: an extra service every 5-10 years, CHF 600-1,500. If you have 5+ automatic watches in active rotation, a winder is more wear than benefit.