If something goes wrong during your time on the Android Pie beta, you can downgrade back to Android Oreo. Problem is, this can be a royal pain in the you know what.
A lot can go wrong while testing pre-release software and there's a good chance the Android Pie beta's issues will force you to drop your device back down to official, stable software. And if that happens, you'll be forced to data wipe your phone.
This is why backing up your data before installing Android Pie is required. If you don't, you could lose all of the data stored on your Galaxy S9.
Backing up your data and restoring said data can take quite a bit of time and there's always a chance you encounter data loss issues when moving from Pie back to Oreo.
If you don't have time to deal with any of this, you'll want to avoid the Android Pie beta and stay put on official software.