When you have a fast and stable internet connection you hardly appreciate how useful it is to read offline docs.

When you don't, it is a godsend.

Definitely. When I got the train from the Gold Coast to Brisbane (1 hour, with really spotty phone coverage) it was my life-saver!

this. I use it on the airplane with some libraries that have online only docs

If you work with more than one language, going to all the sites constantly may be a problem.

Also, dash let you search for i.e. the class and then within the class public interface with just a spacebar.

Plus, it is a native app that you can launch quickly without having to search for the particular browser tab where you current docs is open.

Offline mode for me, mainly, but that also makes reading documentation for the sake of it more enjoyable and practical. I'd casually flip through classes and methods just because they were easy to get to.

It's enormously useful to have all your documentation available in a single place. Along with that, the sidebar makes finding relevant methods and properties very easy.

Loads instantly, + for Mac/iOS docs it is way more stable and usable than Xcode's docs browser.