The full commit message is just as crucial as the first line. Take for example a recent commit to the OBS project on GitHub: https://github.com/obsproject/obs-studio/commit/26d5560da3b7...

The exact commit is as follows:

  libobs: Add scene item grouping
  
  Allows the ability to group scene items.  Groups 
  internally are sub-scenes, which allows the ability to 
  add unique filters and transforms to each group.

Without the full message, I would not know what this commit brought until the changelog is updated on release. Now I know what to expect and why is was committed. In a merge request, I would like to see the same level of reasoning. For example, if there was a new commit pushed in a MR that I already saw that changed one line, I would prefer to see reasoning behind the change.

This is all personal preference though...