Pre-emptive Water Heater Replacement
Cost of replacing it now vs cost of replacing it later plus cost of cleaning up from its failure (anywhere from nothing to a whole lot). Throw in some kind of likelihood of it failing. 

If it's a commercial property, just replace it now. If residential, a home inspector or plumber could probably take a look and give an estimate on likelihood of failure. Any corrosion, strange sounds, etc and I'd just replace it now.