We have a category on our site for impact drivers. There’s an explanation there for what an impact driver does. Basically an impact driver uses a spring to store some of its energy and it delivers that energy in small bursts. This is much more effective for tightening or loosening nuts and, to a lesser degree, for driving screws.
To put it in real terms, with an impact driver you can loosen a nut that would take a regular drill 3 times the size of the impact driver. That’s the big win. But here are the downsides. An impact driver is noisier, heavier, and (slightly) more expensive than a comparable regular drill.
But the bottom line is if you need the maximum torque or if you plan on doing more driving (of screws) than drilling (with bolts) then you should be using an impact driver.