Elizabeth began to think that if Darcy could marry her, it would have been to the advantage of both - how would Darcy be benefited from marrying her?

Mr. Darcy loves her for starters. I think the union would also show Mr. Darcy to have more substance than the pompous condescending aristocrat he is generally thought to be.  Elizabeth would acquire a handsome and very wealthy man from one of the finest families around. As the book progresses Elizabeth comes to care for Mr. Darcy more and more.