The Data Settings page lets you delete all transactions across all accounts in Monarch before a certain date that you can pick. You can also do this for account balances separate from transactions.
The main reason you would want to do this is so you can pick a clean start date where you know all your history is consistent across different accounts going forward. Particularly things can look off on your net worth or cash flow chart if some accounts have shorter history than others.
The culprit of this is because every institution gives Monarch different amounts of history for accounts. Some only allow up to 30 or 90 days and others allow 2 years worth. Monarch always will try and retrieve as much history as possible within the limit of what is available.
If you connect additional accounts to Monarch in the future, they will sync data from before the start date you have selected, and you will need to go back to this page and delete the new data.