Water does more damage than fire, more often
Across residential insurance, non-weather water losses are among the most frequent and most expensive claims. A supply line failing while a household is out for the day can run for hours. Nothing about the house being well maintained prevents a hose or a fitting from failing.
Detection changes the size of the loss
A sensor that alerts you, or an automatic shutoff that closes the main when it detects flow where there should be none, turns a catastrophic loss into a minor one. Several carriers now discount for them, and the discount is secondary to the fact that the claim does not happen.
Prevention keeps you claim-free, which compounds
Avoiding a claim preserves a loss-free discount and keeps the property’s claims history clean, which affects pricing for years. The device is cheap; the effect of not needing it is larger than the discount itself.