(See Oki Wants Ikken Tobi.)
Black 1 comes in handy when 7 gives atari and threatens to make two eyes. If White connects at 3, Black captures at a and has an eye in the corner and one where he captured four stones.
(variations later - please add some yourself)
Black 1 creates a long L group, which White can turn either into seki or (worse) into ten thousand year ko, depending on Black 5. Life with points as in the correct diagram is better than seki (or ten thousand year ko).
If White resists at 6, Black 7 is a tesuji. Black 9 recaptures at 7 and if White connects underneath at a, Black can give atari at b.
Here Black misses the tesuji and ends up with a ten thousand year ko.
Black 1 is badly placed for catching the placement stone 2.
If the marked stone is present Black 1 doesn't work anymore. White 8 is double atari.