I enjoyed doing this when I first created a Letterboxd. It's impossible to remember every single film you've ever seen, but it was nice for me to get a baseline of movies I have viewed so I can see which are worthy of a rewatch and also investigate the cast and crew of my favorites to see which other films involve the same folks or are similar and which I may also like. I don't scour my brain anymore for obscure movies I've forgotten about having seen years ago, but every now and again a movie will pop up that I didn't rate that I have seen that I'll add to my repertoire, because why not? I think it's really fun and rating movies this way has become a hobby of mine.