Beeper needs to be set as your default SMS app for two main reasons:

1. First, import all of your SMS history: This allows Beeper to provide a complete view of your SMS conversations with all your Beeper clients, ensuring you don't miss any messages.
2. To avoid missing RCS messages: Currently, Beeper doesn't support RCS messaging. By setting Beeper as your default SMS app, you can prevent confusion about potentially missing messages that may be RCS-type messages not supported by Beeper.

By setting Beeper as your default SMS app, you can have a consistent and reliable experience with all your SMS messages in one place.