For some applications, it's convenient to specify a path to cache, and have HDFS automatically cache new data added to the path without sending a new caching request or a manual refresh command.
One example is new data appended to a cached file. It would be nice to re-cache a block at the new appended length, and cache new blocks added to the file.
Another example is a cached Hive partition directory, where a user can drop new files directly into the partition. It would be nice if these new files were cached.
In both cases, this automatic caching would happen after the file is closed, i.e. block replica is finalized.