The reason I love this method is because I am harvesting unstressed yeast and I can ensure that I don't over or underpitch. For example, my last brew was at 8.3% (OG of 1.081). I was using WLP002 which was a new strain for me and I one I'd love to use again in the future. I needed ~300B yeast cells to pitch the correct amount so I made a 2.1L starter creating (calculated by yeastcalc) 377B cells. I decanted & pitched 5/6th of the starter and froze the other 1/6th.
Now I know I have anywhere between 20 and 50B cells in a vial, and lets say when I use it, its only 50% viable. At worst case scenario I'd make a 500ml starter then step to 1.5L. Boom, I've got my 300B cells for another 8% brew. If I was going for a beer with OG 1.060 as my next one I would skip the 500ml step and just do a 1.5L starter and I'd have the perfect pitching amount.
EDIT: As I was writing, I realized that not everyone has a stir plate. If you use the intermittent shake method of starter aeration, this method would not be for you as the amount of starters you'd have to make would make it very cost and time ineffective.
