[erlang-questions] when any why to use improper lists

[Karlo Kuna]

dealing with digraph module i have noticed use of improper lists as
representations of edges:
['$e' | 123]

is there a good reason to use improper lists instead of tuple for this and
in general
when is a good idea to use improper lists?? (i can't think of example for
justified use)
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