Jamieson, Fausset and Brown on Genesis 29:8

8. They said, We cannot, until all the flocks be gathered--In order to prevent the consequences of too frequent exposure in places where water is scarce, the well is not only covered, but it is customary to have all the flocks collected round it before the covering is removed in presence of the owner or one of his representatives; and it was for this reason that those who were reposing at the well of Haran with the three flocks were waiting the arrival of Rachel.