I used a Henge Dock for my old Intel MacBook and, other than some scratches on about 40% of the outside of the laptop's cover, surprisingly really liked it. It was a real space-saver for the desk. Though I'd rather have had the laptop screen available in addition to the external monitor, I didn't use it that often at the desk, and the dock was a great compromise.
Fast-forward a bit to me buying an M1 MacBook Air. I've been jealously eyeing the new-ish Brydge Vertical Dock since the M1 design was still housing Intel processors. Brydge apparently bought out Henge, and they're doing slightly fancier versions of the same sort of clamshell docks, but, um, wow did it get expensive!
Good things come to those who wait for the price to drop, and I recently bagged one on Amazon for the M1 for around $30, down from a height of $275!!!
But that's not why you're here. You're here because you might've done the same and you want to know why you can't get your MacBook to display to an external monitor. You've plugged in a keyboard and/or mouse, slammed shift and the mouse buttons, and nothing. Is your HDMI cord bad? Is the Mac still asleep? Is there a setting that you needed to set?
NO!! It's so much simpler!!! You have to have it plugged into a power supply!!
Or, more to the point, clamshell mode doesn't work when the MacBook is on battery power.
Why not when it'll run for 5-8 hours on the battery? And how useful would it be to be able to test that the dock is working before setting up a power supply behind the desk? Very. Or maybe you just want to use the paltry two USB-C ports for something other than power or a dock with power pass-through. TOO BAD!
I have no idea why clamshell on battery doesn't work and isn't, afaict, even a setting. But it doesn't.
I've done this twice now after not using the Brydge for a few months in between. /sigh (Told you using the MacBook with the external monitor is rare.)