West of what?
Assuming they were interested in getting Iran, having Afghanistan would a step in that road. Of course, if they could get Turkey, that would better than Iran or anything else.
But you are familar with chess. The Russians also liked the sport.
This was all about defeating the West.
How was the USSR making Afganistan a puppet about defeating the West? I don't think they were ever so stupid as to embark on something so drastic and damaging without having a namable interest to gain.