Standard zooms, which usually cover the 18-55mm range, are ideal general-purpose lenses. With the standard field of view that equates to the eye at around the 35mm mark on most consumer DSLRs, these lenses go from a little bit wideangle to a little bit telephoto. This makes them suitable for the majority of the kinds of shots that most people take on a regular basis, from scenic views to portraits.
The kit zoom lenses supplied with most DSLRs do a fairly remarkable job considering that they're designed to be as cheap as possible to make, so that they add as little as possible to the cost of buying a new camera. The image quality of these lenses is more than adequate for general snaps but take away the budgetary constraints and it's possible to produce optics of markedly superior quality. If you aspire to produce images of the highest technical standard, possibly at big sizes, and maybe even harbour ambitions to sell them commercially, then you're much better off buying just a DSLR body and forsaking the kit lens in favour of a premium alternative. They'll probably be a lot bigger and heavier but they'll have wider maximum apertures and far superior edge-to-edge resolution, due to the use of technology such as aspherical lens elements and extra low dispersion glass.
Premium standard zooms produce superior optical quality to the kit lens that comes with the camera, enabling them to resolve fine detail with greater sharpness. Their wider maximum apertures also give them an advantage when shooting in low light.
Lens Top Tip
Don't rule out full-frame lenses if you have an APS-C DSLR. Because your sensor is smaller you'll only be using the central part of the lens's image circle but this is where the optical quality is at its highest.