The more you get used to an environment, the more you are prone to loosing track of the changes around you. This hinders innovation and results in biased designs and opinions. One can counter this through many means, such as consulting colleagues, stepping away and coming back at the problem or researching what others have done. Another way to do this is through user testing.
Why I User Test:
Playground to test concepts that does not cost money or effect company
Allows to validate designs concerns coming from leadership and vice versa
Provide evidence based results to sell new changes and other future strategies.
How I User Test:
Figure out what needs to be tested and write a test script around it
Add some extra pre and post tasks to mask the actual task
Create a prototype in InVision or Axure
Mostly I would use usertesting.com, depending on the project i have also done in-person and guerrilla testing
Create a summarized report with key metrics such as, success rate and ease of use with comments from testers.
Overall, user testing has helped me in all stages of UX work. From ideation, to usability and validation.
It is a tool in my arsenal that has worked for me countless times and keeps me close to my research background.