Using a bead head is extremely beneficial in getting the fly to quickly penetrate the water column and sink to the bottom. One of my favorite Smokies flies is a bead head Tellico that is also tied with a generous amount of lead wire creating one of the heaviest flies you can tie in the smaller sizes. When combined with an indicator, it becomes deadly even in the hands of a relative novice. In the hands of an expert, an indicator isn't usually necessary on the small mountain streams and that same expert will catch lots of fish the beginner will miss. As much as weight may be the main factor in the success of the bead head, I believe another factor is important as well.
Flash, sparkle, an attention getter, the bead head is all of the above and this could well be argued to be the most important aspect of a bead head nymph. Sometimes the fish just want to see a bit of glitter and while it may not be pure, it sure catches fish. While some people prefer to stick strictly to natural materials for fly tying, they are once again limiting themselves. Sure, it may take more creativity but as much as I enjoy exploring my artistic abilities, when I sit down at the fly tying bench I want to crank out the most efficient fish producing flies possible. Often this means taking a very popular nymph and simply adding a bead head.
To a lesser extent (or perhaps greater, who knows), the bead head also adds a different element to the profile of the fly. When I see so many extremely flat little critters in the water, I'm sometimes tempted to wonder why the bead head actually catches fish. The large round bulb at the front end of the fly certainly doesn't look exactly like something in nature, but that is part of the game. Often instead of a perfect match, something that is more suggestive works wonders and the bead head is the perfect example. I'm still looking for my first bead head pheasant tail hatch and if I find it, I'll probably make history. In the meantime, this little fly will continue to catch many gullible trout around the world.