What is the purpose of "Carp monitor" option in HAProxy?

[ahmad.badihi]

I'm wondering why should I just be running HAProxy in CARP master node?
What can go wrong if I run it on both nodes with same config and leverage CARP for failover?

P.S. It seems that sometimes CARP failover does not start HAProxy on the new master node correctly, so I suppose it's better to let it be running all times on all nodes.