Although it's usually described as testing the film, it's really resting the whole process.
Everybody's methods differ slightly, and differ in multiple details – choice of developer and dilution, agitation pattern, accuracy of thermometer, the point at which we start the timer, speed of emptying and filling the tank, temperature of the room or our hands, and the cheerfulness of the goldfish, for all I know.
Our shutter, our method of metering and the accuracy of our meter are obvious factors, as is our perception of what constitutes each Zone. (We commonly visualise the Zones as a staircase, but in the real world, they are a slippery slope.) It's easy to add a few more, if we are bored during a long exposure.
If these factors vary randomly, they may well cancel each other out, but it's quite possible that they all tend in the same direction (and our neighbour's details may tend in the other.) Altering the setting on our meter is an easy way to summarise everything, without carrying tables of correction factors around.
All this is mechanical, but we add aesthetic choices to the cauldron. What sort of scene makes us spread the tripod legs? Just how much shadow detail will satisfy us? What do we consider to be a good print anyway? (What does our mentor think of the prints we make now?)
How much do we value ease of printing over other qualities? Do we look forward the adventure of multiple timer re-setting and multiple filtration? Do we value a really good black over other qualities?
Are we printing in the darkroom at all, or are we scanning?
Is the siren call of recreational densitometry tempting us toward the rocks of absolute precision? How much do we enjoy constructing graphs?
All these factors, physical and personal, are summed up in the simple action of turning the speed dial on our meter. A few (more) simple tests to establish our N-0 developing time (and for the really picky, a few more for plus and minus) and off we go.
In practice, until we have made enough mistakes, we don't have a useful amount of data on the problems we wish to solve, and halving the box speed and reducing development by about 20% seem to suffice for almost everybody, so there's no rush.
But sooner or later, it will seem worthwhile to do the tests. Until we've done it, we will have little niggly doubts. "Could things be better?" will sit at the back of the mind until we've wasted those few sheets of film. A well-developed curiosity should be part of every photographer's mental baggage.