[todo-mvp] Why use fragment？

I found that an activity in the project will create a fragment correspondingly. What are the benefits of this? I don't think fragments are necessary.

Start by tracing the activity and its corresponding fragment shown in the issue, then compare their responsibilities and lifecycle roles in the sample. Document the architectural rationale for keeping both, or identify the specific simplification needed; done means the relationship and benefits are clearly explained.

Written by the indexing model from the issue text.