If a water heater drips or leaks onto wood or drywall, then it may cause damage to these or to the surrounding area.
New v. Old
On new homes, nearly all water heaters will have a pan under them, or should. Most older homes do not, however, adding a pan is not a bad idea and if you replace a water heater, then it is usually a requirement to put a pan under the new water heater.
Pan size and drain
Generally having a galvanized steel pan (or other approved pan), at least 1 and ½ inch deep, with a ¾ inch drain will help protect the surrounding area.
The pans’ discharge (drain) line should not discharge under the home into a crawl space or be subject to water being trapped in the line or subject to freezing.
Rusted pans and mold or mildew
Any rusted out or leaking pans should be replaced; when replacing avoid bending bending the pan.
Should a pan be leaking or the drain from the pan be leaking onto the platform or nearby walls, then take a minute and check for damage to the platform or wall. Also look for stains or dark discolorations that may be an indication of mold or mildew. (Read about mold and mildew)