Because, if you are in emergency and need quickly to make just a pot of tea or instant coffee, from the moment you desire it and you can have it ready, it's just in a matter of minutes... not so easily with a fire, which you need to start, to develop it, etc. And mostly is that you want to do it w/o leave a large trace in heat thermal vision domain nor making huge setups that could take long time and could expose you if you are on the run or something. This is said that 90% of population will disappear in the first few weeks after a SHTF scenario, because of lemmings like you!