It provided for numerous checks and balances and principles of federalism in the composition of the Congress. The inherent check of two houses vs. one. Short terms vs. long terms. Equal suffrage of states vs. population-based representation. Election by the legislature vs. popular election.

Also - Great Britain had at the time a powerful upper house in its legislature and everything in the Constitution was a choice to emulate, alter, or reject some element or another of the British unwritten constitution. Assigning GREATER authority to the Senate than the House, as the Constitution does, both made sense from the perspective of the need to empower states, and also reflected the de facto leading character of the House of Lords vs. the House of Commons in Britain at the time. (The Lords not only included many of the great powers of the society by inheritance, but luminaries of the House of Commons very often went up to the Lords in the midst of their political careers whether by creation to the peerage or inheritance of their family titles -- heirs very commonly served in the Commons.)

Over time, of course, the House of Lords surrendered (and after that, was deprived) of virtually all of its legislative authority, and members of the House of Lords became ineligible for service in the highest government posts, such that (today) a politician goes to the Lords only in retirement, where it operates for the most part as a high-profile debating society. The US Constitution tends to lock things in stone, so the Senate's powers remain undisturbed.