Computers are good at repetitive, tedious, fiddly and complicated tasks, and they are good at remembering lots of stuff. As BrowserUK implies, a good IDE leverages those characteristics to pick up errors early and to make it much easier and faster to find the information that is required during program development.

There are a few astounding individuals who seem to be able to hold huge amounts of information about how a piece of code hangs together in their head and access the pertinent information they require with apparent ease. Most of us can't do that. Having a bundle of well coordinated tools that compensate for that lack allows the rest of us to participate in the game and generate quality output where otherwise we would get so bogged down in detail and tedium that we would never produce anything.