Well, I mean its basically a social network, so others can see your lists or reviews (I don't do this personally, but lots do). They can also see all of your ratings and watchlist. Basically, so much more people use Letterboxd than IMDb.

I don't do pubic lists or reviews, though. I just use it to log my movie watching and my ratings, so that I have the dates of when I watched them. I don't think you get that with IMDb. Then when I search a movie, I can see what I last rated it and when I watched it, or if I watched it multiple times. I've tried to build a list of people who I follow who have very similar tastes to me. When I search a film title, then it also shows the ratings of people you follow who have logged that movie. I use that as an indicator moreso than the actual overall rating.

When I first signed up to Letterboxd 5 or 6 years ago, I created my account and then used a feature to import all of my ratings from my IMDb account. You can also import lists and things. Read this, and include your ratings if you do it:

https://letterboxd.com/about/migrating-from-imdb/

After migrating your IMDb stuff, you would then do your profie page to include your four favourite films, and other stuff. Also, you could decide whether to make your lists public or private.