You can study from others results and get the same benefit. (if there was indeed this benefit to be had)
True, you could get lots of tips and rules of thumbs and coach assistant files, etc., through the Forum, and become quite an expert on the game. But being able to test your specific roster in action is taking the testing to a new level, it seems to me. If I could run, say, three variants of my lineup 15 times each before a key game, I would have a much better idea of which lineup variation was more effective. I could also run my favorite lineup in different offensive set many times over and see how my players perform each way and have direction on strategy tailor-made for my specific roster. The difference between the forum and unlimited practice tests is like the difference between learning about how to build an airplane in a classroom or textbook with case studies vs. building an airplane and testing it in a wind tunnel. If this ability was one I had to pay for, this, in my opinion, is like having a way to pay for an advantage. The beauty of the current Supporter system is that it has many nice enhancements that add to the experience of the game, but there isn't anything in the Supporter extras that gives any sort of advantage to Supporters.
Don't ask what sort of Chunks they are, you probably don't want to know. Blowing Chunks since Season 4!