The idea behind this is that, firstly, it allows you to wake up in a more peaceful manner. Secondly, it’s helpful for when you’re sleeping with a partner or in the room with other people (i.e. college dorms, camps, etc.).
To enable silent alarms on smartwatches, you’ll likely have to go into the Settings. On an Apple Watch, for example, you have to turn on Silent Mode and set the timer. When your alarm goes off and you’re wearing the watch, it will begin to vibrate until you turn it off. You’ll know that it’s in Silent Mode because you’ll see a cross through the bell icon.
Fitness trackers offer similar capabilities. The Fitbit Charge 4 will wake you up silently by pulsing three different vibrations along your wrist until you tap twice on the device to turn it off.