There are a number of reasons that you may find it necessary to drain all of the fuel out of your gas tank. The most common reason these days is bad gas. In the old days, "bad gas" meant fuel that was years old, contaminated with water, or was full of solid debris. It was rare to accidentally end up with bad gas in your fuel tank, although there were always reports floating around of people who filled their tank with bad gas right from the gas station's pump. But for the most part bad gas was a problem that affected people like farmers and antique car guys who let things sit for a long, long time then tried to take a short cut by not cleaning the old fuel out of the tank or engine before they tried to bring some piece of internal combustion equipment back to life. That was the old days. These days bad gas has become everyone's problem. The addition of Ethanol to automotive fuel has changed the gasoline game for the worse. Ethanol-enhanced fuel has been causing serious problems in both large and small engines. Where old, ethanol-free gas took years to become unusable, new E10 (10% ethanol) fuel can go bad in just a few months. This is a real problem. Check out this Consumer Reports article
detailing some of their findings pertaining to E15 (15% ethanol mixture) gasoline.
Let's get back to talking about how to safely get bad gas out of your tank before you let it gum up your engine's works.