I have used milestones for more than 3 projects and i've learned the following:
1. Milestones helped me to break large projects into small deliverable projects. This removed the feeling of being overwhelmed by huge projects.
2. I got paid while still working on the same project unlike before where i had to wait for months to complete large projects.
3. In some cases, it becomes easier to justify the cost of the project and avoid under/over charging the client.
4. Gving project updates is also easier.