To provide one possible answer, perhaps Mercury is there to increase our understanding of how the solar system and universe works and thereby employing many talented nerds astronomers to gather data and interpret. Granted, there is no physical law that demands Mercury be there, but there it be, so I sez let's examine it and learn from it.Well, that works for me quite well, actually. It is much like Guillermo Gonzalez's thoughts about the eclipse of the sun when the moon fits so neatly over it - almost as if that was done to help science:
The narrator then introduces dramatic footage of a total eclipse of the sun in North India in 1995, including bystander reactions (clearly, they had not expected the sky to become so dark).
Astronomer Gonzalez enthuses about the event and introduces a surprising fact, well known to astronomers for hundreds of years, but not to many others: The distance of the Earth from the moon and sun happens to be exactly right for key astronomical observations of the sun during an eclipse (400 times bigger but 400 times further away). Of course, the exact position of Earth and the relationship of Earth to our sun and moon are also critical for unrelated biological reasons. Gonzalez sees that as evidence that humans were meant to explore the universe.
Of course, that also implies that God leaves us free to choose whether we want to do science or not.
I mean, if we prefer to run around stark naked during eclipses, killing pigs and screaming that the world is ending, ending, ending, fine. We stay ignorant and we don't know what we don't know. People who can do nothing more than predict eclipses suddenly assume great power, even though apparently it is a simple trick if you know any astronomy and keep good notes.
We won't even notice that nothing important happened during the eclipse that wasn't due to freakouts.
If we take the opportunity to study the solar spectrum, fine too. But then we also need philosophy as well as science, so that we do not lose our way. It is amazing how much freedom God actually gives us, within moral restraints.
Note: I have added Evolution Engineered to "Never a Dull Moment" at left. Fewer dull moments for all!
(Note: The image is from NASA Worldbook.)