There's a mono switch on the monitor bus in Mixbus32C (probably also in Ardour) for this purpose. The idea is to check if all of the main components are still present in mono. It's also possible to made some trade offs with some signals making the stereo version richer while being (almost) lost in mono, if they are not crucial to the production.
I had doubts about this kind of testing. After all, why test in mono in 2018 when we all have stereo ? Still, major commercial productions do this. There are several reasons. In a small coffee shop an owner might have the only two speakers for music right beside one another on the same shelf. Teenagers might share ear buds, one for each, to listen to a new hit. People also listen to music in a kitchen in such a way that both speakers are very close, on a portable kitchen 'stereo' player. In public spaces music can be out speakers that are very far apart.
It absolutely does not matter I think for what I do but I still got into the habit of doing a couple of runs in mono just to check it out. I also have a single speaker for that purpose, just to make it more extreme. Doing so also provides a different viewpoint to the mix.
One video that could be helpful in the general sense is one from Michael White (Whitney Houston, David Bowie, David Byrne, James Taylor, Jimi Hendrix remixes, etc...). This is from a 'Mixing with Mike' session, so it's a different kind of tutorial, much more like sitting in a mixing seat watching what the engineer does.
The LCR approach to mixing (Left Right Center)
Setting levels in general