Following up on the explanations, Jupiter-like planets mark the point where the rocky planets separate from the gaseous planets. The solar system forms when gas and dust begin to swirl and rotate. As the dust molecules stick to each other, they get denser and begin to attract each other. The heavier ones fall to the center of the dust cloud and begin to form the star. Others clump together at various orbital distances relative to their mass and become the basis for the planets. Once the density of the protostar reaches the point of ignition, the star "lights up" and the explosion blows away all the loose dust, leaving the rocky clumps to become the planets (the dust is pushed to the outer limits, probably becoming the Oort Cloud that births comets). The clumps further away become the gas giants, like Jupiter. The "no man's land" between the rocky planets and the gas giants would be like the asteroid belt between Mars and Jupiter.
If a Jupiter-like planet were to have an elliptical orbit, it would sweep up all the inner dust, preventing the inner rocky planets from forming. A circular orbit would allow other circular-orbiting planets to form. That is why a Jupiter-like planet around a star is a promising sign.
-PJ
(I hope I got that right to a layman's degree of accuracy)