Jamieson, Fausset and Brown on Leviticus 17:8-9

8-9. Whatsoever man ... offereth ... And bringeth it not unto the door of the tabernacle--Before the promulgation of the law, men worshipped wherever they pleased or pitched their tents. But after that event the rites of religion could be acceptably performed only at the appointed place of worship. This restriction with respect to place was necessary as a preventive of idolatry; for it prohibited the Israelites, when at a distance, from repairing to the altars of the heathen, which were commonly in groves or fields.