The 6 units in Bury are a 1 turn delay on 1 IPC.
On R2, those forces start retreating (and if the US landed planes there, they get left behind and destroyed).
Japan starts landing in Bury, and moves their China cluster into Sinkiang on J2.
Now those Bury Forces are in danger of being cut off, so they retreat again in R3 to Novo. J3 Japan advnaces to Yakut and reinforces Sinkiang.
Now those 6 units from Bury either retreat to Moscow in R4, or get destroyed.
Stacking Bury does block that 1 IPC for 1 round. But in doing so, they set up a situation where those units can do nothign but retreat or be destroyed all the way to Moscow.
There are other uses for that INF where it actually is a threat instead of just a “reduce speed” sign.