Idea of the Qa5 mainline:
Black: "I have archieved the pawn break that usually black plays after a ferocious fight around move 20 in exactly 1 move. I will build a position without weaknesses and develop my pieces in total freedom, much more than in a ruy or sicilian. In fact what i will get is a sort of caro kann classical mainline, with the advantage over the caro kann proper that white can't play something like the dangerous advance lines ( 1.e4 d5 2.e5 is lame, i can put the bishop outside the pawn chain as in a caro but i can also play c7-c5 in one go as in a french; white has nothing). So in fact this is an improved caro kann, a subtler move order which sacrifices some time to cut white options."
White: "what he says is true, his structure is solid and his pieces can develop to natural squares, but i will give him no time for that since to force this structure he wasted time and his queen is still somewhat exposed. I have a lead in development and i will use this quickly to gain an advantage".
Who is right? nobody knows. Most likely the truth is something between the two points of view.
There are plenty of reasons to believe that the scandinavian is a good opening (including the fact that a lot of GMs play it) , and plenty of reasons to believe that it's somewhat inferior. Make up your own mind; for sure black plan is not illogical, but i personally find black position uninspiring. It's a matter of tastes.
I have never understand the meaning of that opening.What is the benefits for black?