Symbolism, Imagery, Allegory
Besides the fact that they're all characters in Revenge of the Witch, Tom, Tom's Mam, and the Spook have one major thing in common: they're "cack-handed" (1.22). This is serious, almost like being born with an undeveloped twin or a third eye or something. Tom mentions that "they'd even called in the local priest to look at me, and he'd kept shaking his head and telling me that I'd have to fight it before it was too late" (1.22).
Since priests aren't too friendly toward the Spook, it's no surprise that they'd want Tom to fight his magical nature. So what's the deal with left-handedness? Do you have to be left-handed in order to have magical powers? If that's true, we don't ever want to cross Ned Flanders.
In any case, people have long been curious about the odd minds and lives of left-handed people http://facts.randomhistory.com/facts-about-left-handedness.html. Revenge of the Witch picks up this popular trope and runs with it.