When deploying an application, it's not prudent to run through it with only one user to make sure that it works-- unless you only ever expect one user to use it at a time. Web apps often go live after rigorous functional testing only to fail or become unbearably slow when multiple users are accessing it at once. These failures or slow response times could very well spell the difference between success and failure.
Enter load testing.
Load testing and nonfunctional testing are on the other side of functional testing: it is making sure not only that your application works, but that it works well for the intended number of users, and ideally even more. It's using modern load testing tools to simulate real users as closely as possible before you go live so that there are no surprises, and so that you can fix any issues before the application goes into production.
Ultimately, the goal of load testing is to address bottlenecks or other performance issues to improve the application’s performance before those issues occur in production.
Load testing puts users first, ensuring the quality of their experience in accessing the application.