Why use Lumetri scopes?
The scopes are objective and absolute measures of the levels in your image. They aren't susceptible to the vagaries of human perception—colours and levels looking different depending on context—and they work even on un-calibrated monitors.

Scopes are particularly important where viewing conditions aren't optimal - that's why pro cameras will usually have a histogram or scope function. If you're squinting at a monitor in bright daylight you'll have a very different idea of the exposure than if you're inside, and without an absolute measure of the levels it can be very hard to judge what's correctly exposed.

If your viewing setup is correctly set-up and calibrated you can certainly get away without them, but you might find that learning how to read them may give you an insight into the process that you don't get just from looking at the image.

Of course the scopes won't tell you when something looks good, that's your job, but they will ensure that what looks good for you will look good for others.