Can someone tell me what the benefit is of using this cast on technique.

I have tried it and I cannot figure out why people think it is beneficial. I find it difficult to estimate how much yarn I will need and I don't understand why you work backwards to the end/tail of the yarn. Why not start at the tail and work toward the ball?

Am I missing something?

There are at least a couple of cast ons that require you to start somewhere besides at the end of the yarn and work "backward". There is the thumb method that you mention, and the most often used, the long-tail cast on. What I would consider the "benefits" are that they are fairly easy to keep even and they provide a good, sturdy edge, the long-tail method being the best edge. My general rule for knowing where to start is that on worsted weight yarn, using about a size 7 to 9 needle, it will take about one inch per stitch that I cast on. On thinner yarn, maybe 3/4 inch; on bulky yarn, 1 1/4 to 1 1/2 inches. So if you're using worsted weight on a size 8 needle, and casting on 30 stitches, you would want to give it about 40 inches... 30 for your stitches and 10 inches for the tail to weave in later.

There are other types of cast on where you can start at the end of your yarn. The knitted cast on is one. You can watch this YouTube: How to Knit - Knitted Cast On - YouTube. The problem with this is that it is very hard to keep it even, resulting in an edge that looks a little ragged.