When you figure in the cost and risks of lifting against Earth's or the Moon's gravity, including fuel for later in the flight and the vehicle itself, you get a good idea of the benefits of asteroidal material. Further, getting asteroidal materials is not risky business, like launching materials up from Earth or the Moon. The gravity of the asteroids is negligible. A person can jump off any but the largest asteroids with leg power alone. With payloads from asteroids, all we have to do is nudge them a bit so that they pass close to Earth's moon, and the Moon's gravity will divert them into Earth orbit.
Asteroidal resources are unique and diverse.
In general, they are different from the dirt beneath our feet and the ores that we mine from our planet's crust. Likewise, they are much different than the Moon, Mars, and other planets' crusts. Asteroids are much more metal-rich, like the core of the Earth, Moon, Mars and any other planet, and unlike their crusts. The asteroids are normally not as enriched in oxides, silicates, and the other lighterweight elements that floated up to dominate a planetary crust. Many asteroids are like a mixture of a planetary metal core plus a crust plus the mantle in between.