All water heaters eventually develop a leak. It's just a matter of time. A well maintained water heater can last decades, but it too will succumb to rust. Water heater companies, the kind that specialize in just replacing heaters, often recommend replacing a perfectly functioning tank after its warranty has run out. It's hard to fault them for their logic. The damage a potential leak would cause is many times the cost of a new heater.
For the homeowner and the environment replacing a functioning heater is a waste of resources. A drip pan, a giant pan the water heater sits in, is the solution to this problem. The $20 pan will catch any water leaking from the heater and evacuate it from your home. This allows you to run the heater until it actually develops a leak. You maximize the return on your investment in the heater. It is not uncommon for heater to last well beyond double their warranty life.
Besides the economic benefit, the drip pan is an essential safety device. Even if the pan is not connected to a drain, it will retain a good amount of water before it overflows. Leaks develop gradually. There is a good chance that water will be visible in the pan long before the leak becomes catastrophic. Give yourself time to catch the problem.
All around a $20 drip pan is a good investment. Ask your installer about it, or install one yourself. Drip pans are sold at home centers year round.