So that they can cover more people and with that, they are able to have more customers.
For me, I can have native coverage in more areas. There are areas around here where they have "holes" in coverage. In certain areas, everything around it is covered, but in that area, it's roaming.
Now, I don't travel a lot to areas where they don't have coverage, but when I travel to see my parents, I am roaming all the time I'm there. But that's just me in that case.