The 0.15.x branch directly enabled the (0.16.0), which added the Wither boss and command blocks. Without pistons and observers, command blocks would have had limited utility on mobile.
Pigs became rideable with a saddle and could be controlled using a carrot on a stick. minecraft pocket 0.15.0
For the first time, mobile players could use Pistons and Sticky Pistons to create complex contraptions. This update also introduced the Observer block , a Pocket Edition exclusive at the time, which detects block updates and sends a redstone signal. minecraft pocket 0.15.0
to mobile, allowing players to host persistent private servers for friends, even when the owner is offline. Pistons & Sticky Pistons minecraft pocket 0.15.0