Water heaters can and usually will leak, so having a water heater without a drain pan is kind of like having a home without a roof. It doesn't make sense, and it's pretending the inevitable won't happen!
Drain pans come in several shapes and sizes, so there's no good excuse for your plumbing contractor not installing a drain pan below the water heater....
The next, and equally important step is to make sure that the drain pan is connected to an exterior drain. For slab foundation homes, having a drain pan that connects to an exterior drain can be challenging to install, but it is doable!
Having a water heater drain pan that does not connect to an exterior drain is kind of like having a roof that isn't water proof - What's the point?
Make sure your next water heater is properly installed!