The key is that showList is defined on e.g. Int or Char, not [Int] or [Char]. If we tried to do the "obvious" thing for a list, we'd define

    instance Show a => Show [a] where ...

but then we have an overlap any time we try to define a different handler for a specific list of things (in this case, instance Show [Char] would be the overlapping instance we'd want, to produce the String representation). We avoid this by building the representation for lists into the representation for single items (thus, a showList method); we can provide a default implementation of that method in the Show class to do the usual brackets-and-commas representation, using the mechanism intended to allow optimized versions of typeclass methods for particular types, and then provide a specific implementation for instance Show Char without invoking overlap.
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