That's because K got adopted into the Latin alphabet by the crazy Germanic peoples, and now it fulfills most of the functions originally belonging to C - you just think that C is the redundant one because you're a native English speaker.
And because K was quite successful in taking over the C's role in those languages. My native language has no K and it functions perfectly without it. Yes, I am aware that the Roman letter C probably evolved from the Greek K, but that's not a reason to "reinstate" the original one - many other letters did, but you don't see the originals being introduced into written languages today.
I don't think there's a noticeable difference between the usage of C in substantives and in any other types of words, however yes, the letter C is very uncommon in the German language - tends to show up only in recently-loaned foreign words, especially in those of Latin origin. The only exception is the group CK, which is very very common.
Actually, the letter C evolved from the Greek Gamma, not Kappa. It originally sounded like a G, but then for a while Latin when through a phase where the hard C and hard G sounds came to be used almost in free exchange. The letter G (or C cum linea) was invented by a schoolmaster named Ruga (spelled RVCA) who got tired of people mispronouncing his name. (It may have been used occasionally before him, but he popularized it when he taught its usage at the school he founded in around 230 BC. This was the first school in Rome that was open to the public, for a modest fee. Before teachers were just private tutors, mostly slaves, who taught only the children of the very rich) He removed the letter Zeta in order to to put G in its place, since Zeta wasn't really used in native Latin words anyway. Zeta (called Dzeyta and treated as 2 consonants on a row, dz) was later reinstated at the end of the alphabet later, along with Y (or i-Graeca), once more Greek words had been adopted into common Latin usage.
(While G mostly replaced the old pronunciation of C when words were spelled out, the Romans stuck with using C in their traditional abbreviations. In proper names "C." almost always means "Gaius," only rarely the variant of the name pronounced Caius.)
Latin has only hard C and G sounds, not soft. C never sounds like S and G never sounds like J. J is simply the consonant form of I, pronounced like Y. Y (or the Greek I) is pronounced like the German U with an umlaut. In Vulgar Latin C would be palatalized and sound like Ch when followed by certain letters, but Classical Latin never used the Ch, Sh, or Th sounds.
Classical Latin did have the letter K, it was just not used very often. My Latin dictionary contains I think 4 words beginning with a K, all but one of which have more common alternate spellings with a C. I'm unfamiliar with a K ever being used except as the first letter of a word in Latin. There are apparently certain rules about using K instead of C only after certain vowels (when a C would often still be pronounced like a G), but in what Latin texts I've read it doesn't seem like any rules are being followed. In general it seems that Romans would use the letter K only to make a word look exotic.
In Latin the name of the letter C is pronounced like the English letter K, while K is pronounced like Car would be in a non-rhotic English dialect.
Q was originally a somewhat more guttural K sound, similar to the Semitic letter from which it ultimately derives. it had changed to the common modern Engish usage by classical times though. It is very common in Latin, always followed by a u (or v, as these were still the same letter). The difference seems to be that the u after the q would merge with the same vowel instead of staying seperate. "Qui" and "Cui" (its dative form) sound the same except that the former has one syllable and the latter two.
is beautiful, in my humble opinion.
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Thanks for the information.
(Reason #54703 that USamerican spelling is superior. 
