Why is Stockfish suggesting a4?

@DrMonster2135 and @piscatorox

Sometimes the engine gives some great moves in the opening. It is good to look at an opening book, but you shouldn't ignore the engine. If you don't like it's choise, you can look at another move that you like

Can you say Something clear?

In my opinion, a flexible move. White has not decided yet: which piece will stand on the c3-square - a knight or a pawn? It can be expected that Black will develop in the spirit of the defense of Filidor c7-c6 and in this case, move 5. a4 prevents an attack on the bishop (b7-b5) with the capture of black space on the queenside.

True

I understand the computer has calculated nothing wrong with either side. I think 5. h3 by White is stronger. Though because the nature of the game will take long it really doesn't matter what's played this early. They are known as waiting moves in these types of games.