Dubno Maggid: If the Makkot were just a mechanism to free the Jews Hashem could have made the same plague last 10 months. It must be that the makkot were also supposed to make an impression on Bnei Yisrael and everyone is inspired by different events.
Each plague had a specific purpose. What was the purpose of Choshech?
The connection made by Midrash Rabbah between two Chukim - Korban Pesach and Parah Adumah.
Rabbi Jonathan Sacks: When Moshe addresses the people right before they become free his message (which he repeats several times) is telling the story of Yetziat Mitzrayim, passing on the message. Judaism is about ideas and education, not pyramids and ziggurats. Whenever the Torah talks about Pesach it talks about passing on the message.