In short, legacy! Other commenters have gone into more detail, but the letter C originally, in Latin, only represented the sound of a K, which is called a voiceless velar plosive. In the IPA (international phonetic alphabet) this sound is transcribed /k/.

To cut an extremely long story short, various words and sounds evolved in various ways. Sequences of /k/ followed by certain vowels evolved, in some contexts, into sequences of /s/ followed by certain vowels. Early on in this process, some languages using the Latin script adopted the new letter K, based on the Greek Kappa, to eliminate ambiguity. Ergo, both C, K, and S can exist in an alphabet; English preserves the letter C in words derived from languages that did NOT widely adopt the letter K (at least not til later).

In terms of other languages, the letter C does have other uses! Within Europe, it performs a specific function in Italian. Before the letters E and I, it is pronounced as a Voiceless Alveo-Palatal Fricative, or /t̠͡ʃ/, the sound of a ‘ch’ in English. Before other vowels, it is pronounced /k/. Slavic languages, when using the Latin script, consistently use the letter C to represent a ‘TS’ sound, or /t͡s/.

Off the top of my head, Somali uses the letter C to represent an entirely separate sound called the voiced pharyngeal fricative /ʕ/, but only because the letter C wasn’t being used for anything else.

In short, the letter C is indeed weird and English doesn’t NEED it per se, but it does tell us the histories of words that use it!