Okay. So it is not as unusual as I had thought. This is interesting, of course, and it adds another aspect to the understanding of its current use that may be valuable in discovering its origins.
Just to throw in a few more, West Slavonic languages usually use for /ts/ and for /k/. also stands in for /ch/ (as in 'church') with diacritics. Hungarian uses both as well, though I'm not sure exactly what sounds they represent. Whatever it is, it'll be backwards in some way, being Hungarian.
The reason so many alphabets have odd sounds which they use very differently to others, or that are redundant, is because none of these alphabets were originally designed for the languages they're being used for. They're generally adopted from the alphabet next door, then twisted and reshaped a bit to fit a slightly different collection of sounds.
Exactly why English has maintained this particular redundant letter I don't know, though.