When doing MRs domestically, you don't have to explain a thing. You have the constitutional right to travel anywhere you want inside the borders of the USA and its territorial possessions, and nobody can question why you are traveling.
When doing MRs to/from overseas locations, you might have to answer questions from immigration and/or customs officials, or from CBP officers upon returning to the USA. The best advice I can give is to:
A) tell the truth (NEVER lie)
B) answer using as few words as possible (and do not volunteer information)
If you can say "personal" in response to the question "What was the purpose of your trip?" then just leave it at "personal." Don't elaborate unless the officer decides to probe you for details. If he or she does, be honest and say it was for the miles. Trust me....they have seen it before.
Or, better yet, become a GlobalEntry member if you are eligible. That way, you can avoid this questioning in the first place!