Testing for vulnerabilities by manually entering input can be unmanageable. In these days and age where people have low levels of time and patience, the idea of manually providing input to find bugs/holes in a target can be overwhelming.
To reduce this overwhelming problem and save time, fuzzing can be a big advantage. Fuzzing is an automated process where all the heavy lifting is handled by a fuzzing tool. All the analyst has to do is see the response, time, and status code when the process is complete.
Consider a site with many input fields to test for XSS. In the manual method, all we do is feed the XSS payload to the input field one by one, which is too unmanageable.
Fuzzing is the process or technique of sending multiple requests to a target website within a certain time interval. In other words, it is also similar to brute force.