Developer Preview, per say, is not for the everyday users. It is for, as the title says, DEVELOPERS. You know, those guys who write all the fancy apps for your phone? (The reason why the 8.1 update requires a developer account is this).
Basically, when a new platform comes, it usually breaks some stuff. For example, WP8.1 introduces a new way to handle music playback (one of many small features). For this, developers need time to adapt their applications to the upcoming platform. The Developer Preview is basically RTM - features and API is fixed, it WON'T change until release (unless something drastic happens, say, Heartbleed), but small fixes here and there happen. This status is often called feature freeze - no features are added, but existing ones are fixed, and sometimes slightly extended.
So, it does not help Microsoft. It helps the developers get their apps up to date.