If you're a farmer, it takes a certain number of acres to support a family, so many acres for a cow, pig, horse, garden, etc. My ancestor "John" was able to get a grant of land in New Haven, or wherever, that was enough to support himself, his wife and his eight children as they were growing up. But when his oldest sons take on wives, they need farms also. All the land in New Haven is already owned by someone else, so John sells his land in New Haven and buys several parcels of land (enough for all his sons) in whatever open land (usually wilderness) that is available. Migrations took place from coastal places like Boston, and New Haven, and Navigable Rivers, and as each new group came to America new land, generally going in a westerly direction was opened and settled.
There are of course other reasons, but I think the above was probably the most usual reason.
Google the "Great Wagon Road" which was used by settlers to go from Pennsylvania into Virginia, then North and South Carolina and then from those places to Georgia, Tennessee, Kentucky and then on into Arkansas and points west.