Cumulative updates for Skype for Business contain fixes, but also introduce new features.
Here are some highlights:
- Video Based Screen Sharing for conferencing (VBSS) (Jun 2016)
- Offline Messaging (Jun 2016)
- Busy On Busy / Voicemail on Busy (Jun 2016)
- Allow PSTN callers to join a Skype for Business meeting without a PIN (March 2016)
- Group Call Pickup no longer requires Sefa util (Nov 2015)
Check this knowledge base article for the complete list.