Why do we use the letter 'g' for a 'j' sound?

I actually just read something about this... apparently "j" and "u" are new letters so, it would appear we are actually using the letter "j" for a "g" sound.I believe j was derived from "i".

Both 'y' and 'j' actually have related patterns of development, from what I understand. 'Y' entered the romance languages originally as what they still in certain contexts call a 'greek i', in that it used a traditionally greek character to take on certain 'foreign' types of sounds. Likewise, the 'j' in the same tradition has developed to accomodate the space between the hard 'g' (as in golf) and the traditional latinate/germanic 'j' (an 'i' with a 'y'-type sound like in the names of Roman gods and in the name Jung, for instance). So where a language like Italian still uses a softened G (Giovanni) like others use a J, we've opened up to the greater phonemic range that the extra letter gives us. If you think of names and their origins, the difference is instructive, names of northern origin tend to have a 'g' whether hard or soft (Gerold, George, Gary), while 'J' names tend to fit the softened 'G' sound and come from the South (John, James, Jacob).

Same reason spanish speakers use a 'y' for a 'j' sound. We're all crazies!!!

The Latin /g/ became an affricate /j/ before front vowels in Old French. So the Old French word “gentile” was pronounced with /j/, but in Latin it was pronounced with /g/.

With the influence of Old French orthography on English, the convention of spelling /j/ with the letter “g” before the vowels “e” and “i” was adopted for words borrowed from French or Latin.

But for words of Germanic origin, like “get” and “give”, we use the hard /g/.