In my personal view, Alexa is a standout amongst the most famous web ranking system in this decade. In spite of the way that the Alexa positioning does not reflect web traffic precisely, it is still imperative for website admins to enhance the rank. I have web that get more than 20,000 visits a day, and only got rank more than 200,000. That's maybe because Alexa rank the website by gather data from users who access the web and have Alexa Toolbar in their browser. My web visitors mostly use mobile phone.
Last month (October 2015), I tried to use their pro service (Basic Plan) to get my website certified. It need 3 days to my website certified in the Alexa dashboard, and took almost 1 month to show to public that my web is certified. In their certified metric, my web increased it rank significanly from 210,531 to 27,639.
So, my conclusion from my personal experience is: Alexa Pro can increase rank for website with more traffic but have less visitors-with-Alexa-Toolbar.