You can keep fuel in the tank for 1 - 2 days, but if you keep too long the nitro content is gone and heavy content of oil is left. The fuel become no good; very hard to start, powerless and produce lot's of craps ( unburned oil )
Second reason, you don't want to leave some fuel inside the engine either because it's taboo
....... it's sanitary reason that keeps your engine last longer.
On Road : Xray NT1 , RX8 , T4 || Mugen Seiki MRX4-R , MTX4 , MTX3 prospec || Tamiya M03 , M04 || Top Racing Sabre FD2
Off Road : Xray XT8 || Hobao Hyper 8
Radio : Futaba 3PKS || KO propo EX-10 eurus