This piece of workflow was copied directly from our internal deployment gem.
Load balancing between availability zones is made on DNS level. This means that in case
of ELB configured for 2 availability zones, 50% of traffic goes to the first one and 50% to second.

We discovered problem, when you have registered ELB for 2 availability zones, but instances are registered
only in one of them, then 50% of traffic lands in vacuum (requests time out). The responsibility of adjusting availability zones
enabled in ELB is mitigated by AWS console GUI, so that's why you did not have to bother.