Why to use hook_drush_help() when we have 'description' in hook_drush_command()?
The answer is context. 

The command data pulled from hook_drush_command() is used when listing of all available drush commands with the command:

drush


So if a user is curious, "What drush commands do I have, and what do they do?" hook_drush_command() would provide that snippet. This could be useful for a short description in a directory of commands.

If a user is curious about a specific command:

drush help my_command


That's where hook_drush_help() would be more applicable as it can provide a detailed description that is only about that command queried.