While the preference to read the manga over watching the anime may be opinion-based, there lies within that some facts readily understood outside the anime/manga community.
Some people prefer to read the book instead of watching the movie.
- Perhaps they don't have access to the anime/movie.
- Perhaps they prefer manga/books as a medium, maybe avoiding their experience of the manga/book being "corrupted" by watching an anime/movie adaptation of it.
- Perhaps they feel the need to enjoy this hobby in private and they can't watch an anime/movie with adequate privacy.
Some people prefer to read the book before watching the movie.
- Perhaps they prefer to know the manga/book to educate their anime/movie experience.
- Perhaps they have greater access to the manga/book than the anime/movie.
- Perhaps they couldn't wait to read the manga/book before there was an anime/movie adaptation.
Some people prefer to read the book after watching the movie.
- Perhaps they were unaware of the book/manga before watching the anime/movie.
- Perhaps they use anime/movie adaptations to educate their selections of manga/books.
"If they can't make an anime/movie adaptation, is it worth it for me to read the book/manga?"
"That anime/movie had an interesting concept. I wonder how the book/manga treats it."
- Perhaps reading is difficult (learning disability, non-native language, *et al.*), so watching an anime/movie adaptation reduces the learning curve and gives them greater access to the manga/book.
Some people prefer to watch the movie instead of reading the book.
- Perhaps they don't have access to the book/manga.
"I have trouble reading because I have dyslexia."
"I'd rather read the manga in Japanese, but it's hard for me to find it."
- Perhaps they prefer anime/movies as a medium.
- Perhaps they feel the need to enjoy this hobby in private and they can't read the book/manga (or store it) with adequate privacy.
When the material is in my native language (English), I prefer to read the book before I watch the movie. However, when the material is in a non-native language (Japanese), I prefer to read the manga after I watch the anime, using the anime almost like one would Cliff's Notes. But those are my preferences and their justifications; they may not be yours or your friends'.
This isn't an exhaustive list of justifications or reasons. Feel free to add to them if you want.