Maybe you got distracted by the latest episode of Desperate Housewives and the bath overflowed. Or maybe your dishwasher hose sprung a leak whilst you were at work. Some reasons for flooding the house are preventable, some aren't.
If you're lucky, you'll end up with mildly soggy carpet. If you're unlucky, you'll have to evacuate your house for a few weeks whilst your insurer sorts out the mess. And if you're really, really unlucky, you didn't have home insurance at all.
Installing a drain in any room that could unexpectedly flood. This could include the laundry (due to a leaking washing machine), the bathroom (due to an overflowing bath or shower) or even the kitchen (due to a blocked pipe that serves the dishwasher). In most cases, the drain can be installed out of sight anyway, so you won't even know it's there (e.g., under the washing machine or in the dishwasher cavity).
Whatever the reason, you'll be glad you installed the drain when the unexpected happens - think of it as a once-off insurance.