See: Koppa (letter)
Or, see what I have written on the subject:
The Phoenecian alphabet was adapted for Greek more stupidly than we might think. The greater dumbness occurred with san; but koppa also owes its short tenure to archaic Greeks being slow on the uptake.
Phoenecian had a velar plosive, kap̱ (Hebrew kaf, כ), and a uvular plosive, qôp̱ (Hebrew qof, ק). When the Greeks adopted the Phoenecian alphabet, they took both letters on. Greek does not have a uvular; but the /k/ before back vowels was pronounced slightly retracted, as one would expect: [ḵ]. So the Greeks spent a couple of centuries writing /ko/ and /ku/ as ϙο, ϙυ; this happened throughout Greece (Jeffery 1990:33). Gradually, though, Greeks realised that [ḵ] and [k] are the same phoneme, and should be written as the same letter; while some Doric regions held on to koppa into the fifth century, it did not survive the switch to the Milesian alphabet.
Since koppa does not represent a real phonological distinction, it is only used in transcription of inscriptions, not in linguistic discussion of dialects. It does not appear in lexica, for example.