When you use projects, you can create estimates, orders and bill for the tasks grouped in the project all under one order.
Once you have completed all the tasks, you can invoice the customer at that detailed level or use a higher summary level as part of the invoicing.
As it can take more than one person to complete the tasks to complete the project, plus any additional resources (people and things) add to the price and all can be billed to the customer.
Facility Management allows you to keep track of what the estimate was versus what the final billing cost. Simply: