Thanks for following up, @Santo_Kum!
The HA proxy takes the place of a cloud-provider’s load balancer. It may be less applicable now, but at time of writing, a load-balanced public IP from GKE would cost you $30/month, whereas a little VM running haproxy could be had for ~$2.50/month 
(Of course, you could just use the cloud provider’s load balancer, and disregard HAProxy)