She is probably the only one that could tell them exactly why they were there. While her soul was the same age as the rest of them, she was blessed the gift of time, if you wanted to call it a gift.
The Silver Millennium took place about the same time as the dark ages on Earth. The culture was very regal at that time despite what we find recorded in the history books. Just as time can be altered, so can the history that is remembered. That was part of her gift; being able to bend time and to change the roads that people remember. The up rise of the Earth Kingdoms against the Moon Kingdom was a tragedy that Queen Serenity did not want anybody to remember of than those that could change it, if it were to happen again. The Queen had called on her that day, just prior to her last breath.
All the years that she spent at the gates of time and watched, she saw the creation of the senshi. She also saw why there was the need for them to exist. The Earth had lost its guidance. Even with the first coming of the creatorís message, the people refused to acknowledge him and went on with their immoral ways. There were sexual indiscretions, humanity violations, abuse of authority, and more going on in this world that was created for peace.
The creator then handpicked 10 beings to place them in a guardianship over his people on the Earth. The first time she had seen the birth of the senshi, she was shocked. Shocked to know that it was the creator that made them and not the mythical gods of the Greeks or Romans. However, just as Lucifer himself was corrupted when given power above others, these too became corrupted. They were still inherently good and protected the Earth and those around their leader, but they were corrupt.
Farthest from the others with the least communication, Setsuna was possible the least corrupted, but even she fell into the godly worship that the others had. The Earthlings started it when they saw Minako transform. The noticed the planetary symbol and associated her with Aphrodite. Yes, the planets feed their powers, but that is what the creator supplied them. They can harness the planetís power and use it to keep peace and defend that which is good.
Once it was seen that the senshi used the power of the planets, they were called by their planetary names and their Greek or Roman name.
It is a shame that through the ages, the senshi have become gods to the people and the people remain lost for failure to look at the real Savior.