What are you wanting it for?
Currently I have a galaxy watch 4, but I've also gone through a couple of fitbits over the years.
The ones I've had, to a greater or lesser extent track steps and heart rate, estimate calorie burn, stairs and active minutes. Have varying exercise functions - where they track time, miles, calories burn etc, depending on the exercise, including being waterproof for tracking a swim session, my galaxy tracks lengths in the pool, and works out metres swam.
They also track sleep, and my galaxy can (dubiously) measure stress, body composition.
That's the main 'use' of them.
In terms of motivation - they can help you move more in prompting when you've been sat for a while, or just reflecting your current steps and that being motivation to do more, and improve. But you have to want to improve - and you use the features to remind/prompt/track progress.
Other features include phone messages and notifications coming through and being able to reply, and even answer calls. Reminders, timers and media control on your wrist, and payment.
Their corresponding apps are also good.
I'm happy with my galaxy - but there's some things my fitbit did better - battery being the main - I charge everyday whereas my fitbits lasted 5/6 days. But the galaxy has much more power hungry features - I can answer a call for instance instead of just knowing my phone is ringing, Bluetooth my headphones straight to my watch and have a Spotify playlist downloaded on it, so could leave the house without my phone.