Agile promotes continues and early delivery of a viable product. The process demands that you can tolerate changing requirements. In Waterfall, the final product could be developed over the course of months or years, and you would not see a working product until the end. In Agile, you want to have sprints or iterations that produce meaningful value to the customer that you can show.
In software development, the requirements can change easily and frequently. Also, software is very easy to adapt to changing requirements compared to modifying a physical product in manufacturing, for instance.
Agile is not applicalbe to every part of every industry, but it can be adopted by small teams. For instance, if you have an assembly line, then the line itself works on a Waterfall process. However, developing the idea for the product can use the Agile process before it makes it to the assembly line.