I've used multiple displays back when monitors were still pretty small in size but i can't find a good reason anymore to use more than one for a developer in the age of 27"/5K. It's more distracting than productive.
And I find multiple monitors are extremely useful. Especially if your monitor is only 27’’.
I'm very happy with my built in monitor on my 16" Macbook Pro. Brightness, HDR, local dimming, retina and 120 Hz. I have yet to find an external monitor with all those specs.