The display shown is a common OLED display 128x64 pixels and is capable of being programmed to show a range of characters and graphics using several Arduino libraries for SSD1306 based displays. 

It isn't essential to have a display, but for many options it's useful to have and these displays use very little current when blank, having no backlight. Basic usage is displaying the RTC time and last transmitted measurements such as cycling through solar statistics or current temperatures.

Software to implement these ideas can be viewed on github here