When discussing distances like the size of a galaxy cluster, or a supercluster, or a void, the megaparsec is handy … just as it's handy to use the astronomical unit (au) for solar system distances (for single galaxies, 1,000 parsecs – a kiloparsec, kpc – is a more natural scale; for cosmological distances, a gigaparsec ...

Why are parsecs useful?

It's the largest unit of distance. To measure distance of astronomical objects, astronomers usually measure the parallax angle. The parsec can be used readily with that data. So it makes calculations easy.

What does per megaparsec mean?

: a unit of measure for distances in intergalactic space equal to one million parsecs.