We find that some of the plagues and earlier tricks used to sway people and/or Pharoah were performed by Aharon rather than by Moshe. Hashem specifically told Moshe on those occassions that Aharon should be the one to effect the specific plague.
It is true that these situations were symbols of gratitude - the earth had protected Moshe when he struck the Egyptian, so he hsould nto strike the earth. The Nile had protected Moshe when he was a baby, so he should not strike the waters, etc.. So Aharon performed those.
But it also shows us that the specific person is not so important. Moshe, in this case, was chosen to lead Israel out of Egypt because of specific qualities he had within him. But Hashem bringing these plagues via Aharon instead of via Moshe shows that Moshe was just as replaceable as anyone else. Hashem was not bound to Moshe.
The saying goes, "The graveyard is full of people who thought they were irreplaceable" (it is sourced as "Unknown").
Moshe was important and was an important part of the plan and an important part of the exodus from Egypt. But Hashem shows that if necessary, other people can fill the spot. It was not dependant on Moshe.
Aside from this being a lesson to Moshe or to each one of us individually that we are replaceable, it was a message to Pharoah that Moshe is leading the show here, but he is not really so important. If you think you can knock Moshe off and derail the whole thing, he is easily replaced.