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Safety - This is undoubtedly the most important reason. Gas water heaters require air in order to operate. When flammable liquids, such as gasoline, are stored or spilled near a gas water heater a huge fire or explosion could occur, and since water heaters are often installed in the garage, basement, or a storage area, these situations can often go undetected until there's a problem.
After 2005, gas water heaters have been built with Flame Vapor Ignition Resistant (FVIR) technology. These water heaters prevent the breakout of flashback fires by trapping and burning the the hazardous vapors within the body of the heater instead of the surrounding area. They are able to do this, in part, because the combustion chamber is completely sealed.
https://www.waterheaterleakinginfo.com/water-heater-stand/