Both of my kids will only need one phase- my older kid got hers when she turned 11, and we are still waiting (getting 6 month checks with the ortho) for my 10 year old. It just depends on the problem so not all kids will need 2 phases.
The reasoning for 2 phases is that adult teeth tend to migrate back to where they came in so if the baby teeth are moved to the correct position before adult teeth come in, the adult teeth will move less later on. But again, it just depends on what the need for braces is.