Azure is providing the service. Azure NetApp files is Platform as a Service (PaaS). Because it's PaaS, we don't know (or care) what the service is running on. That's what we are paying for. I don't want to worry about creating VMs to host the service, updating the service, etc. I just want to use the service.
When you create a new NetApp Files volume, you give it a name and a resource group to use. That's it. You don't create VMs, add storage, etc. You just sign up for the service, and they take care of the details. Now you can start using your storage.
Dedicated subnets aren't just for Azure NetApp Files,
"Subnet delegation enables you to designate a specific subnet for an Azure PaaS service of your choice that needs to be injected into your virtual network."
It could be any Azure PaaS service. Azure WebJobs, Azure Cosmos DS, Azure Storage, Azure Cognitive Services, Azure Boards, Azure Pipelines, etc. There are dozens, if not hundreds.
So any service that can benefit from having it's own dedicated subnet can benefit from a delegated subnet.