Here’s Google engineer Jeff Sharkey describes this “richer access” in a Google Plus post:
We heard loud and clear that developers wanted richer
access beyond these directories, so in Lollipop we added the new
ACTION_OPEN_DOCUMENT_TREE intent. Apps can launch this intent to pick
and return a directory from any supported DocumentProvider, including
any of the shared storage supported by the device. Apps can then
create, update, and delete files and directories anywhere under the
picked tree without any additional user interaction. Just like the
other document intents, apps can persist this access across reboots.
This gives apps broad, powerful access to manage files while
still involving the user in the initial selection process. Users may
choose to give your app access to a narrow directory like “My Vacation
Photos,” or they could pick the top-level of an entire SD card; the
choice is theirs.It looks like Google has taken the – mostly negative – feedback to heart
and make microSD card access more flexible, while keeping the
security measure in place for apps that have no place in modifying
data across the microSD card. The responsibility of deciding at what level
of access an app is granted falls to the user, which is not ideal for user because
it could lead to confusion, or at worst, exploitative behavior by
shadier apps. But this compromise is still preferable to the lockdown
from KitKat, which prevented many legitimate apps from working as
intended.