It's easy as moderns to look back on the past and oversimplify and also to import our own assumptions. The gods of the Greek world, for example, were not the same over time and place. An interesting case might be made for example with Aphrodite who had a huge variety of different roles and was worshiped in very different ways by different groups of people.

The correspondence that you point to was partly due to a different ideology held by the ancients which tended to assume that gods of different cultures that performed the same roles were the same gods. An assumption you can clearly see in the writings of Herodotus or, say, Tacitus' Germania.

Early Roman identity is murky and largely fictionalized, making it hard to pinpoint just how much is "Roman" in their religion and how much is "Greek" or "Etruscan." This civilization did not come to being in a vacuum, Roman religion developed in constant contact with it's neighbors and was shaped by that contact.

However, there are some interesting points to be made about your final question. These gods were not a 1:1 correspondence. For example, Mercury is a god of commerce and money, and that is one of his important cultic functions in the ancient world. Hermes is not. Some gods, like Apollo, are direct borrowings, but most have a more complicated negotiation of identity. The Ares-Mars connection is pretty interesting. While Ares is a well attested god in the Greek world, he's generally pretty negative and there is little cult. His Roman counterpart has a lot more dignity. Aphrodite and Venus are also very complicated and the identity of Venus qua Venus changes significantly over the course of the Roman empire. The Romans also adopted a whole lot more gods than the Greeks did (or than the Classical Greeks did - there's evidence for an expansion of Greek cult in the later periods).