This from famed Physicist and Astronomy professor, Michio Kaku...
http://online.wsj.com/article/SB10001424052970203517304574306131239654294.html
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Jupiter, 318 times more massive than the earth, acts like a cosmic vacuum cleaner, sucking in or deflecting debris left over from the solar system’s birth 4.5 billion years ago. If it weren’t for Jupiter’s colossal gravitational field, we wouldn’t be here, since the earth would be hit with deadly comet and meteor impacts every month or so. Most of the U.S. would just be an empty graveyard of bleak craters.
The bad news is that a comet impact could happen to us. A black eye for Jupiter would be a body blow to the earth. We got a taste of this back in 1908, when something the size of an apartment building plowed into Tunguska, Siberia. This “city-buster” flattened 100 million trees with the force of a hydrogen bomb. But this recent Jupiter comet, much larger and coming in at perhaps 100,000 miles per hour, would have unleashed the power of hundreds of H-bombs. It might have engulfed most of the East Coast in a huge firestorm, triggering a massive tsunami and destabilizing the weather.
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Jupiter’s real benefit to us is probably in the fact that it helps corral the asteroids into a fairly stable asteroid belt. (with help from our little buddy Mars of course)
I believe what happens is... Depending on their orbital periods, these things have orbited the Sun hundreds of millions, even billions of times, just like the earth has. Sometimes, when they cross the orbit of Jupiter, Jupiter is nearby, sometimes it’s not. On those occasions Jupiter is nearby, it perturbs the asteroid/comet’s orbit a little bit, bending it toward the planet, so that on its next close pass, it passes even closer to Jupiter. Over billions of years, if it hasn’t already hit the sun or an inner planet, it gets drawn nearer and nearer to Jupiter until... BLAMO!... Jupiter eats it.