
Other than touch-based screen gestures like tapping and swiping, the Digital Crown and Side buttons are the Apple Watch's only control mechanisms. You can use the Digital Crown to scroll through lists and zoom into pictures and maps, plus it can be used to control slider bars like volume and font size. The Side button is the Apple Watch's one-step access to contacts on your favorites list who you can then call, write a text, or even send your heartbeat.

The Digital Crown and Side buttons have a few additional uses that you may not already know about. We've got a list of 10 important actions that the Apple Watch's external controls activate.
Activate Siri
In addition to saying "Hey Siri," you can activate the personal assistant by holding down the Digital Crown until you see "What can I help you with?" You will also feel a tap on your wrist, acknowledging that Siri is listening.

Trigger Apple Pay
When near a supported terminal, you don't have to open the Apple Pay app on Apple Watch to activate it. Simply double-tap the Side button to bring up your card information. Then hold it near a terminal to make the purchase.
Click here to read more...
Article Link: 10 Things the Digital Crown and Side Button Do on Apple Watch