Control over what is materialised is required to define permitted access to data for security purposes.
In a distributed system, data access is controled at the IPC or networking level.
When a view provides a subset of the data using a non-injective function, it may be important for that subset to be materialised on a different physical machine - i.e. so only the data defined by the view is sent over the network to that machine. This is one of the great features of logical independence.