I have not seen your set up. But it depends on the position. Wing pawn moves like a4 or h4 have purposes, it just depends on the chess position. These are the possible reasons of a4 move.
1. Stop or slowdown your enemy' queenside activity.
2. Provoke a weakness on your opponent's queenside. Which can give you active piece play there.
3. Fix a weakness there.
4. Possible tactical blows.
5. If the position has an opposite side castling. Your opponent wants to open the position there for his rooks to pressure your king.
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" Deep calculation is not what distinguishes the champions. It does not matter how far ahead you see if you don't understand what you are looking at. When I contemplate my move, I first must consider all the elements in the position so that i can develop a strategy and develop intermediate objectives"
-- Garry Kasparov--
"Tactics must be guided by strategy"
--- Garry Kasparov--
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