If I wrote an explanation then what would check that explanation? Maybe I write "we use a Tree instead of a Wobble because Wobble doesn't support the APIs we need". But then maybe when you come to work on it, it turns out that the current version of Wobble does support those APIs. Maybe it's actually better at them than Tree. Whereas if I have a unit test around Tree that exercises the kind operations that we need to do, then you can just try dropping Wobble in there and see for yourself whether it works or not.