Positional Tactics

Just as one can use combinations to mate the king or win material, one can use tactics to gain positional advantage.

What are some examples?

Here's a position #87 from Lev Alburts Chess Training Pocket Book. It comes from this Petrosian / Gligorich game in 1959.

Since most of the puzzles in Alburt's book are examples of tactical standards, I devoted my attention to the jammed-up kingside and whether I could find a way to take the N on g5 and somehow extricate the B at h4. However, the answer Alburt gives for this puzzle is positional:

87. Cutting Off Reinforcements

1. a4!. Now the Black Queen's Rook and Bishop will be cut off from the
rest of his forces for a long time. Otherwise, Black could play 1. ... b5 and
later, at the proper moment, ... a5. Now on 1. ... b5, White has 2. a5, and
on 1. ... a5, he has 2. b5-- in both cases sealing the queenside. (Petrosian,
T. - Gligoric, 1959)

I was impressed by Alburt's answer, but when I pulled up the actual game I discovered Petrosian had not played 21. a4, but 21. Nc4. So a4 must be the result of post-game analysis.

So I fed the position to Fritz 12 I found that a4 doesn't even show up in Fritz's analysis until you set it to examine at least eight candidates, then when I left it to run all night, a4 marched up in Fritz's ranking to the third highest, but even so it was scarcely different from the other moves in the top five.

I like Alburt's "a4!" but is it really that good a move? Or does this position illustrate a classic hole in chess program analysis?