You have a website that has its traffic and produces sales, you want to increase its productivity and so you do some split testing to see which landing page converts better. Split testing is a good way to improve your website's productivity but the conversion ratio of a website depends also on the kind of traffic you drive to it. You can get free traffic or pay for it, the first requires your time and effort while the second make your wallet lighter: you want to spend less as time or money and get more signups, sales and so on. You have to analyze the conversion ratio of every traffic source of your website and to do this you have to use the querystring to have a different sales tracking for every traffic source.
I do an example to make you understand better the concept i have expressed before:
let's guess that you buy 1000 visits from a company but at the same time you get traffic from Adwords, Google, your email marketing and so on and so you have to separate the sales from the 1000 bought visits from the others. Ok, to do this you set this link for the 1000 visits you bought:
now you have a script that stores in a mysql database every sale and in the field 'source' will put the querystring value that in this case is 'companyname'.
When you have spent all the 1000 bought visits then you query the database for all the sales which source is 'companyname' and so you get the conversion ratio of your website on that traffic source.
Conversion ratio is a very flexible factor, split testing shows it but also varies depending on which kind of traffic you drive to your website.
Hope this post is helpful...