HA and scaleability
As just a single server host is easily powerful enough to handle all but the highest workloads, high-availability (HA) is really more important aspect than the traditional scaleability. Simplest and most common form of HA is a dual server solution.
When the user and device count goes to tens of thousands of users, or more, the web portal might require some boost in the form of an additional server host or VM. Additional server hosts can easily be added one at a time to a HA solution when needed. Same goes for other server roles like database, analytics, monitor, and backup in the solution.