The reason for doing usability testing is supposedly well known, but sites still screw up, so perhaps the reason for doing it is not clear to everyone.
The point of usability testing is to get the site seen through the eyes of someone who is not familiar with it.
You can do a lot worse than read this book on usability testing. It is Don’t Make me Think by Steve Krug. It is very clearly written and easy to read (which is good, given the subject matter) and you will learn a lot from it.
If you want to dip your toe in the water, you can get a flavor of what it is like to be a usability tester yourself by visiting tests at fivesecondtest
You can have your site looked at by other visitors and you give back to the community by testing other people’s sites.
Their ‘classic’ test gives you five seconds to list what is noticeable about the sites that are presented to you.
Doing those tests taught me one big lesson, which is that if you as a visitor don’t know in an instant what the site is about, then those five seconds go by in a flash while you are trying to figure it out.
And that means you just don’t ’see’ the rest of the page.
Whereas if the purpose of the site is clear, whether through the name of the site, or through the tagline, or whatever – then the rest of the information on the page simply makes sense more quickly because everything has a context.
I guess the lesson of usability testing is to beware of falling into the trap of thinking that the site is crystal-clear to everyone. It might only be clear to you who has built the site and already know what it is about and how it works.