Why does a watch need to be sealed properly?
The best known benefit of a reseal is to maintain the watch’s water resistance.
A wet pressure test is performance after a reseal to confirm the watch resists water to its rated pressure in static conditions.
A 30 metres/ 3 bar water resistance rating or higher will be sufficient for general everyday use, resistant to splashes and rain when the watch has been resealed.
Watches rated 50 metres/ 5 bar and above are guaranteed resistant enough for swimming and showering if resealed.
Though the most useful benefit of a reseal is the resistance to dust and moisture.
Without a reseal the caseback and crown are able to harbour small pieces of dust/ debris that can stop them closing flushly against the case.
The gaps in the seal that this causes allow more dust and also moisture to enter the case.
When these reach the movement gears they are liable for slowing or even coming to an abrupt halt.