Some updates become available only after installing another update. You can back up with Time Machine, for example.
Improves compatibility with certain email servers when using Mail.
The OS X Yosemite 10.10.5 update improves the stability, compatibility, and security of your Mac, and is recommended for all users. The OS X Yosemite v10.10.5 Update is recommended for all OS X Yosemite users.