Was thinking today about the point of bringing a book to something, when I will usually always have something to do either that requires no props or just on my phone to kill time with, and realised that it's a really specific cultural signal of "I am prepared to spend some time alone here." (Similarly filled by "I have my knitting", "I have my sketchbook", etc etc)
Going to hospital to visit someone where you aren't the primary visitor? "I can just sit in a corridor whilst you and your son talk in private... I brought a book!"
Going to have to wait with someone while they do an important task? "Don't worry about it, just get on with the Task, I'll have a book in my bag so I'll be happy as a pig in shit."
Arrived somewhere before your companions so unavoidably need to sit at a station? "I didn't even realise you were late, I was just in my book." Etc.
Even though logically, they will know that I could probably entertain myself for that time by talking to a friend on messenger, or staring out the window, or catching up on work emails, having a book in my hand is the reassurance of "I didn't expect you to be making me the centre of your attention, I am not in any way surprised or offended by you focusing on your own needs right now... I have a book."