So without the SD card, You have a standard boot to the B&N Nook, with standard screen and subscriptions you MIGHT be able to still do (Is Nook still in existance?)

WITH the SD Card, you have an unlocked Android tablet. Notice (black screen list picture) there are a TON of settings that the Cyanogen Mod gives you!  If you understand Mods, I'm sure there are newer updates.