“A” depends on nixpkgs.
In that case, if A import nixpkgs { inherit system; } or accesses nixpkgs.legacyPackages.${system}, both are functionally the same. This is true but realistically, as flakes get more popular, few projects will have only a dependency on nixpkgs.
A depends on B and nixpkgs, B depends on nixpkgs
Now if both A and B import nixpkgs { inherit system; }, there are two instances of nixpkgs instead of one. In further cases where the dependency tree grows larger, the number of instances would grow linearly with the number of dependencies.
In contrast, if both A and B access nixpkgs.legacyPackages.${system}, and that they both depend on the same nixpkgs flake, there will only be one evaluation of nixpkgs.
For this case to work, another requirement is to use the inputs “follows” feature, so they are both pinned to the same version of nixpkgs.