Oh ok, I didn't understand that followers could use spell tomes with mods, thank you for that extra info!

It's intended for duplicating unique one-time (or otherwise hard to find) spell tomes for followers to use (as some mods allow them to learn spells from tomes). Otherwise, I'm not really sure what the uses are. :)

I don't understand what the point of creating a spelll tome of the spell you already have is?

i use this all the time. Very usefull