As a web application grows and gains popularity, more and more users use it. Depending on the success of your application, it probably won't scale at the same speed as new users adopt your application.
The problem is not only a problem because of the increased number of users; due to the relative success of your application, users will naturally ask for new features, increasing the interaction with these users and your application.
Most scaling and performance problems don't give an advanced warning; they just suddenly appear one day. If you don't have a plan to scale your application, you'll probably have to work hard just to keep it responsive.
The truth is that most developers don't design the application to scale during the initial stages of development, arguing that performance and scalability should be deferred until the application is in production.
However, what is the difference between performance and scalability. The difference is described...
