Sometimes the user could experience choppiness or other issues on a small portion of the project. This could be because of various reasons such as low RAM/Hard disk availability, other heavy GPU related process running along with PRE, HEVC or another media specific issue, etc.
In these situations, the user might want to render that specific part alone and not the whole timeline. The user might also not want to turn off GPU acceleration as that would mean them having to render the whole timeline.