It’s simple, really. Prior to Android 4.4 KitKat, applications —
provided they had permission to access the SD card — could read
and write to any area on removable storage, including the system
folders like DCIM, Alarms, etc. That has all changed, and now
third-party applications — as in ones you download from Google
Play or elsewhere — can only write to files and folders that they
have created or have taken ownership of.
This keeps things “tidy.” Apps aren’t dumping files everywhere on
the card — something we’ve all encountered — and instead have
one central location to put all their files. There also are some
serious security concerns that were addressed by not letting an
app write files just anywhere.