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I looked in the source code and it seems it's set by RecurringOrdersScheduledTaskAddIn, the scheduled task that creates recurring orders:

foreach (RecurringOrder recurring in RecurringOrder.GetRecurringOrdersForScheduling())
{
    try
    {
        Order order = Ecommerce.Services.Orders.GetById(recurring.BaseOrderId);
        if (order is object)
        {
            //Logger.LogGeneral("Recurring Order", "Creating recurring order based on orderID: " + order.Id)  'TODO: Fix logging in general
            OrderManager oManager = OrderManager.GetFor(order);
            oManager.Recurring();
            recurring.LastDelivery = recurring.NextDelivery;
            recurring.Save();
            //Logger.LogGeneral("Recurring Order", String.Format("Recurring order process succeeded for order with orderID: {0}. Delivery date is {1}, next delivery {2}", order.Id, recurring.LastDelivery, recurring.NextDelivery))  'TODO: Fix logging in general
        }
        else
        {
            //Logger.LogGeneral("Recurring Order", String.Format("Creating recurring order based on orderID: {0} is failed - order was deleted.", recurring.BaseOrderID))  'TODO: Fix logging in general
            result = false;
        }
    }

It's also used to calculate the next and future delivery dates inside RecurringOrder which are then shown in the UI for a recurring order:

So yes, it seems it's being used to determine when the next order is placed, based on that date and the recurence frequency and unit (i.e. ever 7 days, 1 month and so on).

Imar