A client's Files tab can contain dozens or hundreds of files, so it's important that files are organized by task, so that files are easy to locate.
The same applies when creating a task from an email - any files that email contains will be associated with the new task.
But when an email comes from a client that is not associated with a task, the files in that email are placed in the General files section of the client's Files tab. (This is also where you can upload your own client-related documents that are not associated with a particular task.) When too many files end up in this section, individual files can be hard to identify and locate.