I used to believe it was just a marketing ploy BUT we were told that it is not fully true and that the nutrition breakdown is a bit different. My DS saw a hospital-based consultant paediatrician and it was he who advised that when his milk intake dropped to around 20ish ounces per day, that we switch to the 2nd stage milk as it had more calcium for LOs who were not hungry enough to take all of their milk AND have solids. If she is still taking good amounts of milk then you can stay with stage 1 but if you are worried her milk intake is low, then the later stage milks do have some different nutrition break-downs.
I do believe there is a good part that is marketing, but I do also trust that there are benefits to moving on to the later stage milks for some babies.