Do I need DisplayLink software on my Mac? I just upgraded to Mojave and my second monitor would not work. Tried all kinds of things suggested in the top Google results to no avail. Then I read to try and update DisplayLink, which I had not heard of.

I downloaded and installed it but got a security error saying I had to allow it. I thought this can't be a necessary piece of software, since it's not even by Apple. I ran the uninstaller, rebooted, and my second screen worked instantly, even remembering that the application bar should be attached to that screen.

Why was DisplayLink installed to begin with? Why should I need it? My second monitor work fine with just native macOS Mojave. So what benefit am I missing if any?
DisplayLink is most commonly used to connect external displays via USB. It is proprietary software, not made or supported or automatically installed by Apple.

If you had it installed earlier, it was because you have installed it yourself at some point in time.

Ordinary monitors connected via HDMI, DisplayPort, mini-DisplayPort, DVI or similar on Macs work with multiple monitor support without any extra software.