If I’m traveling to another country, I might want a local number so that people in that country can call me from without incurring a long distance call charge.

Also, a lot of 2 factor verification systems have trouble messaging international numbers. A lot of the airport Wi-Fi’s I try to log into, for example, almost never succeed in sending me a pin on my international number.

Adding a roaming data plan to my existing number only solves a small number of the problems international travelers face. And even those problems are almost always resolved at a higher cost than getting a local SIM.