As discussed, there are a bevy of reasons why people might watch the movie adaptation first.
However, by doing so, you miss out on a one-time joy that you’ll never, ever be able to replicate again.
Experiencing a story free of bias.
When you read a book for the first time, your imagination fills in the details. Your idea of a setting & how characters look will inevitably differ to another reader’s.
Reading is an act of creation. Every time you open a book, without prior experience of the world contained within, the reader creates a textual palimpsest; another layer of fictionality that watching a movie robs the reader of.
Remember – a movie is one person’s interpretation of the source material. Watching that, without having read the book, robs the reader of their creative agency – and from a reading perspective, that’s quite tragic.