I think that the disulphide bond is in itself pretty weak. But when used as a bridge linking polypeptides, it makes the polypeptide on overall a stronger structure than it was to begin with.
For example- melting sulphur:
Crash cooling of sulphur generates enormously long chains of "plastic sulphur" from the 8-sulphur-ring (or part rings as is common when you heat sulphur).
Secret to better rubber:
sulphur bridges hold the rubber units together, thereby increasing the strength of the rubber. This is the secret of vulcanising.