Greetings Peach,
As the experts, who had already answered observed, this is purely a business process consideration. If your (Client's) organization supports a "streamlined" process in which direct Order creation is beneficial you can go for it.
As you've written "Since the Creation of Maintenance Order will generate Notification, we were thinking that creation of Notification is redundant if the system allows to create order and notification in one transaction." I assume you are still using Notification and Order integration. But as mentioned, this is optional as well.
If you choose to create a Notification in the background, you can still registed breakdown times for MTTF/MTBF calculations, Catalog Codes for tracking and any additional information once the Orders are created.
But consider this: How is work requested, released for execution and reported? If you onyl create Orders, you have only Orders to go on. Therefore, you might require more verbose User Status management, and you may find yourself with Orders that are being created unnecessarily and have to be closed, Orders being released prematurely... You have to have a very "flat" process around this immediate Order creation.
On the other hand, consider that having Notifications created prior means a more "deep" process and more overhead, but you can have a step to filter works to be released on the Notification level before Orders are created, if you have more than one Notification for the same work, you can assign it to a single Order, and you have more options when designing roles and authorizations, since you can separate access and business transactions for Notifications and Orders for your Operations and Maintenance users, if necessary.
Please ask further questions if you have any!