The 32768Hz crystals are normally more exact than normal processor crystals (the precision depending on ambient temperature) and a big reason why they are so much used for time keeping is that the low frequency is the result of a quite large/heavy crystal. A large crystal is less affected by age. If you get some extra particles inside, it will not affect the total weight as much so diffusion will not give as high drift as with faster crystals.

They are normally optimized for use on a wrist, in which case they may manage one or a few seconds/month. You normally get worse performance when not in a wrist watch because of the different ambient temperature. But they are still hard to beat when you need a good time source in an off-line application. Next step up is normally to integrate a GPS or other radio receiver to receive reference time.