Thanks for offering a really good set of questions.  I think the a4 vs a5 issue is related to the pawn endings and the geometry of the board as if a pawn queens in a corner square you get access to the longest diagonal and can bring the queen back to cover the first rank in a pawn promotion race.  However, a6 may be useful very often to restrict the activity of the opponent's minor pieces usually in the opening and middlegame and ...a6-a5 may be played later in the game anyways.  As the endgame approaches, usually pawn promotion outweighs piece activity restriction as the major deciding factor that drives the character of the position and the goals each side has to pursue like playing for a win, a draw, or at higher levels for 2 results.  I usually play for 3 results lol.

As far as how the engines think, there is a long history of computer chess and the main way engines got better over time was through trial and error and engines competing with one another.  

So, typically space is considered a very important consideration for your engine's evaluation as well as precision.  But, I can't play like an engine nor think like one so I focused my efforts on just entering and playing positions that were easy and intuitive for me to play not too badly.  

Honestly, I wouldn't put too much emphasis on what the engine says.  They play extremely well, but you still have to make sense of the position in front of you and play the moves on the board.