Why use the manual when you can just use the UESP or better yet the forum. The only time I'll look at the game manual is when I'm 1st starting out so that I get acquired with the controls.
It helps in game, it helps those that don't have access to the internet 24/7, it helps those that don't spend free time online ( I remember those days. )
It shows fans that the company took pride in the game, that they have an artistic side.
That they're not too cheap to publish one.
That they don't care just about releasing everything with an expensive game guide.
It helps players that want to plan ahead but don't want spoilers, it helps P&P gamers, it really just helps a fair few people tbh.
Edit: ms typing and that I forgot about toilets.