| The mobile site tester addon lets you test with google to see if google considers your site mobile friendly. This is important because of the mobile friendly update that came out in April 2015
This addon lets you load a list of sites, and then test to see if they pass or fail the google mobile friendly tag test. In addition to pass or fail data it lets you find out exactly what is wrong if something is wrong.
It even gives you a url score as well as weighting metrics so that you can see what is the largest problem with the site. The google online test tool does not even give you these weighting metrics, which makes this addon extremely useful as you know what to focus on fixing first.
Not being mobile friendly can affect your search rank, which is of course a problem. Plus you want to be mobile friendly anyway, so that users/customers can use the site and so that sites make money on mobile devices as well. | 
