It is not about scale as much as the possibility of getting apps from multiple sources. Each source can set its own curation policies, and each user can choose the sources they want. And for things that people want to keep out of their life, lots of people don't want NSFW images on their phone, but they want easy access to all the apps they want. What is NSFW in one country could be totally fine in another country (e.g. images of people wearing tank tops or swimsuits).