What's the deal with Q?

Question

In all forms of Latin I know, the letter Q is always followed by a U. No other letter seems to be bound this way. The combination QU stands for something like /kw/, and it would make more sense to me to let the letter Q alone stand for /kw/ and absorb the letter U into it since they always come together. This would not be the only letter to stand for two sounds; consider X. The only point in the letter Q seems to be that it signifies that the following U is consonantal, as opposed to C whose following U seems to be always vocalic. For example, consider the difference between qui and cui. This makes Q look like a special case of C, but weirdly so, as it only modifies the pronunciation of a following U, not the C itself.

What is the origin of the letter Q? Why does it behave in such a weird way, always requiring a U? Why do we need the letter (in its current form) in the first place? Can the origins of Latin pronunciation or alphabet shed some light on the unusual behavior of Q? In short: What's the deal with Q?

Answer 1

This answer already gave the answer as pointed by sumelic.

In short, in archaic Latin, there were three letters 𐌂, 𐌊, and 𐌒, corresponding to C, K and Q. All those letters were used for both the sound /k/ and /g/. Later the letter G was used for the sound /g/ so C was the only letter for /k/. The letter K became rare and remained only before /a/ (Kalenda…) and Q, which was used in Etruscan before rounded vowels /kʷ/, remained only before /u/ and was therefore only conserved in QV.

Cf. also Wikipedia:

The Etruscans used Q in conjunction with V to represent /kʷ/, and this usage was copied by the Romans with the rest of their alphabet. In the earliest Latin inscriptions, the letters C, K and Q were all used to represent the two sounds /k/ and /ɡ/, which were not differentiated in writing. Of these, Q was used before a rounded vowel (e.g. ⟨EQO⟩ 'ego'), K before /a/, and C elsewhere. Later, the use of C (and its variant G) replaced most usages of K and Q: Q survived only to represent /k/ when immediately followed by a /w/ sound.

Answer 2

From a synchronic point of view, the retention of Q as an allograph of C had the advantage that it disambiguates the distinction of the very common words QVI (qui) and CVI (cui), which would otherwise be homographs.