STP Activity When the Network Remains Stable, what is the purpose of changing the bid?

Okay, so I know BID plays apart in helping switches to designate a root switch ect. My question is, after everything is established and you have your root ports, designated ports, non root switches and root switch, why does (look at step 2.) the BID receive get changed into THAT senders BID before being sent off? Is it to keep switches from trying to establish "dominance" again orrrr?

Step 1. The root creates and sends a Hello BPDU, with a root cost of 0, out all its working interfaces (those in a forwarding state).

Step 2 .The nonroot switches receive the Hello on their root ports. After changing the Hello to list their own BID as the sender’s BID and listing that switch’s root cost, the switch forwards the Hello out all designated ports.

Step 3. Steps 1 and 2 repeat until something changes.