A laptop may be enough for development. However, it feels very comfortable if you have a desk with a large screen (or screens) and perhaps an external keyboard

A laptop may be enough for development. However, it feels very comfortable if you have a desk with a large screen (or screens) and perhaps an external keyboard and mouse that you can all attach to the laptop for heavy duty work. (As well as external hard drives and whatever tertiary storage you may need.) If you are going to carry it around, make sure to back up religiously. Laptops often tend to be abused and thus have a noticeably shorter lifetime than desktops. And in some cases, as people have mentioned, you may need a desktop simply to get as much power per buck as you can -- depending on your project.