From Wikipedia:
The hammer-forged barrel has a polygonal profile with a six-segment right-hand twist. Each depressed segment within the interior of the barrel is the equivelant of a groove in a conventional barrel. Thus the interior of the barrel consists of six smooth arcs of steel rather than six sharply defined slots. The stabilization of the round is not by conventional rifling, utilizing lands and grooves, but rather through a polygonal profile consisting of a series of six small arcs connected by flat surfaces. The method by which Glock barrels are rifled is somewhat unusual; instead of using a traditional broaching machine to cut the rifling into the bore, the Glock process involves beating a slowly rotating mandrel through the bore to obtain the hexagonal shape. As a result, the barrel’s thickness in the area of each groove is not compromised as with conventional square-cut barrels. This also has the advantage of providing a better gas seal around the projectile as the bore has a slightly smaller diameter, which translates into more efficient use of the combustion gases trapped behind the bullet, greater consistency in velocities, increased accuracy and ease of maintenance.

So, with all of that said, why?
Easy. DO NOT EVER use lead ammunition in your Glock, regardless of caliber or date of manufacture. Lead is a soft metal and will expand, trapping the rapidly expanding gases in the barrel resulting is a catastrophic malfunction. I spoke with a police officer and fellow instructor who had a barrel rupture the week before we spoke. He was unhurt, but the possiblity of injury is huge!

The big reason I bought the 22 was for round interchangeability; you can buy a Glock 22 and change out only the barrel and mag then shoot 9mm, 357 and 40. In a situation where law and order breaks down, one must use what they find. Having a gun that you can change caliber’s on, that is light weight, and the mod parts are even lighter is a handy thing. Plus you can switch to your 9mm for range plinking practice. I don’t know of any other handgun that can do this.

The 40 S&W Hollow points have similar stopping power to ball 45 colt, which is impressive for the recoil; they’ve been known to leave ash-tray sized welts in their targets.

I would suggest taking a look at the articles here, as they may help answer your question: http://www.learnaboutguns.com/category/information-for-beginners/

If not, my contact information can be seen here: http://www.learnaboutguns.com/2008/03/19/contact-the-author-of-this-site/