If you've ever seen the benefit to writing a main method to test some small piece of code for school, unit testing is the professional/enterprise version of that same practice.
Also imagine the overhead of building the code, starting your local web server, browsing to the page in question, entering the data or setting the input to the proper test seed, submitting the form, and analyzing the results...vs building and hitting the nUnit run button.
Here is a fun image too...
I found this image here...:
http://www.howtogeek.com/102420/geeks-versus-non-geeks-when-doing-repetitive-tasks-funny-chart/