Just to chime in ...
I have a cadence sensor on one of my bikes that broadcasts to my Garmin watch - which I bought for a loss less than most 'smart' bike computers.
I don't find it useful - but novice cyclists might if they haven't yet got a feel for when they need to change gear?
I use the watch for logging my rides. I'm still using an old Garmin Etrex Vista HCx for navigation. The work flow for setting up routes is a bit pants, but it is useful having a display on the handlebars. This is where the watch really falls down to be honest.
Now that I'm cycling more, and trying to get fit, I find knowing what heart rate zone I am in useful. Especially when I should be doing a recovery ride or saving my legs for a longer / harder effort at the weekend ... it is a good reminder that I need to slow down. I think this matters more with age too, when recovery can take a lot longer. And stops the inevitable burn out of doing too many hard efforts without enough recovery.
I don't have a power meter - I can't justify the cost at the moment, but I have been tempted to pony up for one. Why? Because at the point you are improving fitness, it would be good to see that progress (power output versus heart rate). But you can also measure that progress in other ways - for instance looking at average speed or time taken for a route/segment that you regularly ride.
I'll probably get a cycling computer next year and maybe, if things are going well, some power meter pedals but these are essentially both luxuries I could live without. The watch on it's own, combined with the old Garmin Etrex are good enough.