What is the open file operation, what closes the file operation, and what is the purpose of the introduction of these two operations?

The so-called generation of documents is to copy the relevant directory table of the file to the area specified in main memory, the establishment of a file control block, that is, the establishment of the user and the file connection.

The so-called closed file is the user declared that the file is no longer in use, the system copies its contents of the file control block in main memory to a file directory entry on disk, and releases the file control block, thus cutting off the user's connection to the file.

The purpose of introducing this action is to:

Save memory, reduce access time, and speed access.

What is the open file operation, what closes the file operation, and what is the purpose of the introduction of these two operations?