Laze guns interact with the sheilds causing basically the destructive power of Tsar Bomba+, but knives/swords (when used properly, as Gurney Halleck taught Paul to do) pierce the sheilds. Read the book nerd.
It has been a while, but I'm pretty sure the bladed weapons are needed because the shields specifically block objects with high kinetic energy. I also recall one of the houses on arakis getting bombed, but only after they were able to turn off the protective barrier. I might be mixing up Sci fi books, though.
You’re not - it’s pretty explicitly the point. The technological arms race has gotten so advanced that everyone has devolved into fighting with knives.
The movie had its problems and I've never read the books but even I know the answer is because the shields don't stop slow moving objects otherwise you wouldn't be able to pick up a cup. It's even explained in the film, admittedly very briefly.
But then again that's also how personal shields work in Stargate
Also a laser hitting a shield causes a massive explosion, which could be at the shield or laser gun. Bullets get stopped and a laser would likely kill either user if many kilometers away, or both of closer.