I bought it mainly for the screen size, but the multi window feature is nice.
Others use the s pen a lot more than I do. They use it for actually taking notes, I prefer to open an app and type.
I mainly use the s pen for creating photoshop type pics in s note.
It also has the gimmicky features such as smart stay and rotate. The camera has cool features like only showing the color blue, green, or red and yellow when taking pictures.
TWRP is a custom recovery. With it you can do nandroid backups which backup the entire phone and you can restore that backup if you need to do so.
It is also used to restore certain parts of the backup such as the kernel and system. It also clears cache, davlik, factory reset, fix permissions, and of course flash new ROMs or mods.