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?