The process of creating an image only for the purpose of displaying a gradient is inflexible, and is quickly becoming a bad practice. Unfortunately, at the time of this writing, they very well might still be required, but hopefully not for much longer.
Most mobile devices have a LED backlight or LED flashlight. The better the the display, the better the game experience. More and more smartphones have nowadays a OLED display, that saves energy and the battery lasts longer.