I've been watching an unhealthy amount of Ramsay's Kitchen Nightmares lately -- not the trashy Fox reality drama production, but the U.K. version, which is more focused on the food and, y'know, kitchens. Anyway, while watching the interactions between the various members of the kitchens depicted, it's interesting the parallels you see between the various degrees of chefs and how that correlates with the various degrees of tanks within a raid operation.
Without getting too, er, French, the two biggest fish in a kitchen are the chef de cuisine (or, as the rosbifs may call the job, the head chef) and the sous-chef de cuisine. This correlates directly with most tanking hierarchies you'll find, where there is a main tank and an off tank. (In some guilds, there may be a number of off tanks.) Much like how the head chef is the boss when it comes to what's being served, the main tank is in charge of handling the burden of developing the tanking strategy and executing it. And likewise, while the sous-chef is second-in-command and chief substitute, the off tank handles a similar role within their own structure.
Similar to how the head chef might not be the one always sitting there and actually preparing every single dish, the main tank isn't always the one on the boss. For both, their personal strengths might lend them better to a different, lesser role while the lieutenant gets a chance to step up and carry some of the burden. You see this in fights where one tanking class is stronger against a particular mechanic than another, like perhaps putting a highly mobile warrior on Shannox's dog Riplimb. Essentially, there is more to being a main tank than the jobs you perform. It's also the experience, the leadership, the dependability, and the prestige.