The d2 constant is a bias correction factor. The within group ranges are pooled by averaging. So this average represents the typical short-term variation. But it is not sigma. It is proportional to it, but it is biases. So d2 corrects this bias. Dividing by d2 removes the bias.
The same idea is used in most of the other control charts.
Any textbook about SQC / SPC that covers control charts will explain it.