This gets applied when users are placing a scheduled order, since you can only add additional prep time to ASAP orders and on SCHEDULED ORDERS the user is entering the Date and Time so we cannot override that by adding additional time. You can use this function to add more time to scheduled orders.

How to add default preparation time?

Control panel > Configuration >Settings > Edit settings 

Click edit setting and a tab will appear on the right right which will have the field named Scheduled Order Time (highlighted above) here you need to enter the additional time for Scheduled Orders and hit save. Make sure you publish changes.

How this works from a customer's standpoint:-

Now when the user schedules an order the system will add the Default Prep Time (30mins) and Scheduled Order Time (45 mins) to the Current Time and give the earliest time available as explained below. 

You can see the system is showing error because the user is trying to place an order before the earliest available time which is 4PM. How earliest time is calculated (Current Time + Default Prep Time + Scheduled Order Time)

2:44 + 30 mins + 45 mins = 4PM (Times are in 5 mins increment)

Below no error message as the selected time is validated and falls within the requirements set.