Stuff like this gets overlooked all the time, but the ones who tried it haven't gone back.
A second screen might seem a waste of money and space – you already have one, after all – but if you're a power user, your workflow will benefit so much from this.
One advantage, probably the most apparent one, is the increased screen estate. You can keep two apps open at the same time with no compromises – i.e. no half-screen apps that can't fit etc. You can watch a tutorial about something and work on it at the same time. Or you can work on a website on one screen and see the changes live on the other. The possibilities are endless.
Ok, suppose you have a second screen at home. That's cool, but you lose all the advantages when I go to work or when I work on a train, bus, etc. That's where technologies such as Sidetrak come into play (which coincidentally is the model I have 😛).
With a portable second screen, you can use your laptop as always, but you'll have a produting screen on the right of the screen of your laptop, sacrificing one USB port. You attach four sticky magnets to the back of the screen of your laptop and then you attach the external device. It's a bit heavy, though, so be sure to check if your computer can bear the weight, or it may break the lid.
It's still a novel technology – in fact, it has many rough edges that need polishing – but it's already in a fairly usable state. You'll need a laptop of the right size, though. The screen itself averages about 13" among all manufacturers of this kind of device.
The device itself doesn't connect through HDMI – it uses a proprietary technology named DisplayLink that allows this and many more portable external monitors to receive both the oiwer and the audio/video signal through the USB port. A downside of this is that you get limited software support for older releases of Windows and macOS.
If you fancy bigger screens, alternatives exist. They're less, umm, sticky, but they require a surface to be put on. I don't have one of these, but I've seen these average 15" to even 17".
I feel like people underestimate the productivity boost a second screen provides, though I've been seeing more people using more than one screen (maybe it's the people I follow?)