Let assume you have a infinitely scaling algorithm implemented in your application, using replication, ECP, or any other means of horizontal scaling, and let assume you know how to run your system under any volume of requests, the trick is to deploy required number of computing nodes in the cluster. If we are talking about cluster of 2-4 nodes your administrator (or as they call it today "devops engineer") will install anything manually. Probably he will easily handle 5 nodes configuration in the cluster. But what if you need to deploy 10, 100, or 200 properly configured, interconnected nodes?
Devops are lazy, if there is some easy way to automate task then they will automate, even if there is need to use newer and fancy tool. [Though requirement here, this new and fancy tool shold be easy to learn: they could spend a couple of hours reading StackOverflow or vendor forums, but certainly have no times for several days. Thus simpler way usually wins