MSG files are encoded in a proprietary Microsoft format called RTF. Modern email clients all support inline display of HTML -- but not RTF.
Unfortunately, RTF to HTML conversion isn't perfect. Therefore, the only way to ensure the MSG file is viewable in its original format is to include the RTF file (appears as a separate attachment).
For users more concerned with speed, enabling this feature allows you to view these RTF files as converted HTML directly in the original email, bearing in mind some minor formatting losses may occur due to the imperfect nature of RTF to HTML conversion.