full and healthy bookshelves perform a similar function to room treatment and (more importantly, imo) makes a space cozier and more comfortable to be in.
i have nothing against professionally treated rooms - they sound amazing - but i personally prefer a comfortable space surrounded by the things that i love and value to some sterile, professionally treated, Serious Listening lair with a single chair and 50k in electronics on the floor surrounding the obelisks from 2001: a space odyssey. i could hit the lottery tomorrow and would still gladly make that sacrifice (or alleged sacrifice) in sound quality 8 days out of the week 😂