Turning a Fitbit tracker off is different from a restart or a reset. A restart will turn the tracker off and on, within one action, and you will not lose any data. A reset will delete all data and return the device to its factory settings. These instructions are for turning a Fitbit off and then turning it on.
Not all Fitbit models turn off. The ones that can are the Fitbit Blaze, Fitbit Ionic, Fitbit Versa, Fitbit One, Fitbit Surge, and Fitbit Sense.
The ability to turn off a Fitbit shouldn't affect your decision when choosing which fitness tracker to buy, as very few people will actually need to do so. The main features you should look for when buying a Fitbit are: battery life, tracking features, and the overall appearance and style of the device.