From a performance perspective, trainer inertia aside, altering cadence can shift the muscle fiber type recruitment a bit. But mostly, I’m concerned with 1) riders holding power close to target power in order to really challenge their aerobic capacity, both at the muscle and more centrally, at the heart & lungs, and 2) making things transferable to the way they’ll ride or race outdoors, especially as they move through the Base/Build/Specialty cycle.
As far as point 1, slow twitch fibers are capable of producing relatively low amounts of force but fatigue very slowly while fast- and “mid”-twitch fibers are the opposite–more force as you work your way up through the fiber type continuum but greater sensitivity to fatigue. So if you turn the pedals slowly, with higher force, you’re recruiting more fatiguable fibers and also fibers that rely more on anaerobic metabolism and hence produce lactate as well as muscle acidification. The lactate is a good thing, as long as you have an aerobic system robust enough to process the excess before the hydrogen that accompanies it acidifies the muscle and stalls contraction. But even then you’re shifting your energy usage to more costly anaerobic turnover (more glycogen for less ATP) while remaining more aerobic–a quicker spin with lower-force contractions–yields far more ATP per glucose molecule cleaved from your muscles’ stored glycogen.
This is a more detailed than usual explanation of the “oxygen cost” of ATP (frequently referred to as “aerobic efficiency”, not to be confused with mechanical efficiency which is also a concern when you pedal poorly at high or low cadences) and how it differs between aerobic and anaerobic energy production. And on top of this, the more intense contractions take a greater toll on the muscle tissue (and not just the obvious pedaling muscles) which can see you fatiguing earlier for reasons beyond energy depletion.
All of this is to say, you make better use of your energy stores (and intake) and keep fatigue at bay when you spin a lighter gear more quickly than grinding a bigger gear. But sometimes grinding is a necessity, and training as such, often or occasionally, depending on your cycling discipline and time of season, is advised and built into each plan and governed over the course of each BBS cycle.