I think the very first line in the novel shows that the long history of the estate with the respectable Dashwoods as owners was a great point of pride for the elder Mr. Dashwood. Since the division of the large estate or the sale of parts of it could be a possibility if he left it to Henry Dashwood outright. Perhaps what he saw of his nephew, how his nephew felt about his daughters or how strong of an influence Mrs. Dashood had over her husband even made him think that was very likely to happen.

Then, if any of the girls were to marry and had part of the estate as a dowry, it would pass to the control and ownership of their husbands, unless it was legally secured in some way.