7.23 BGP Bestpath Selection - Always Compare MED

I was able to get the lab working but my question is:

Why do I need the bgp always-compare-med on R2, SW2, and SW4.

Since it is configured on R3 and R5 they are only going to pass their best route to the rest of AS 200?

Before configuring bgp always-compare-med on R2, SW2, and SW4, I already see that the prefix from AS 300 will be used?

Thanks,

Shawn