If you've ever wondered why astronomy uses strange units like the AU (astronomical unit) and the solar mass, here's the answer.
Until recently, neither the AU nor the gravitational constant were precisely known. However, a determination of the relative mass of another planet in the Solar System or of a binary star in units of solar masses does not depend on these poorly known constants. So it was useful to express these masses in units of solar masses (see Gaussian gravitational constant). Today, the AU is extremely well measured using interplanetary radar and G is well measured, but the solar mass persists as one of astronomy's arcane historical conventions.
Basically, it was easy to calculate the relative mass of one object compared to another even though no one had a precise value for the gravitational constant or the distance between the earth and the Sun. However, even though we now know that we these values to high precision we're still stuck with the old units of measurement.
