If you have a mag failure, it may just quit and act like it's not there, but it can also do things that will make the engine feel like it wants to leave the airplane and move to another city. That is, really rough running.
If a mag just quits working completely while in cruise, chances are, unless you are paying really careful attention at the time, you won't even notice it, and the surprise will come at the next runup.
The typical in-flight failure you WILL notice is sudden banging and vibration that makes you jump out of your skin. As one example, a traditional mag has an internal plastic timing gear driving the distributor. If the teeth are worn down, the gear might skip a bunch of teeth, advancing the timing way out of specification although it still may still fire the plugs, and you'll feel it in the cockpit.
Severely advanced timing will likely be damaging cylinders, so you want that magneto off now.
You aren't going to have a simultaneous double magnetor failure, where everything goes quiet due to lack of ignition, in any normal universe; the statistical probability is too close to zero (unless the plane has one of those single-drive dual bendix mags and the drive fails).
What's going to happen is one will quietly stop working (like a coil failure), which likely result in you carrying on unawares, or you might start to notice a subtle skipping as one mag misfires randomly (internal capacitor failures can do that), or, it will fail in an active manner that results in crazy roughness, like a major timing shift as I described.
You want to disable the bad mag ASAP, so you immediately select the individual mags to identify the bad one, and carry on with the ignition switch selected to the good one.
And if the roughness was actually caused by something else, you've confirmed the mags are not the problem, and carry on with them at BOTH.
