They work well if you’ve got mudguards (else you’ll get soaked by water being thrown up at you) and are wearing “normal” trousers. Regular fabrics stick when they get wet, and that bit on top of your thigh isn’t a great place for things to stick with all the movement as you pedal. Quicker and easier to put on than waterproof trousers, and you can have them rolled up around your waist if it’s not raining now but looks like it’ll pour soon.
When I did a lot more Brompton miles, I kept my set in the bag along with a light waterproof jacket and that was good enough for most downpours in normal clothes.
If you’re wearing proper cycling gear, not a huge benefit really.