It activates Cassidy in case you didn't pick her at the start, meant for those who picked her on their first date but had to redo their saves.
Viewing post in Honeymoon Chapter 2.1 Update is out now! comments
I assume you know where the save files are but for general purposes:
If you're in windows, go to the search bar and type "%appdata%" then go to the "Roaming" folder, click the folder "RenPy" and "inheathoneymoon".
Then move the file "persistent" to another location as a backup.
"persistent" has all the data that stays on even if you delete saves. Hope that helps!