The reason "new" letters are intoduced is usually that the sound already exist in the language but is written with a different letter that sound less like it. eg: 'Yaveh' / 'Javeh'
This post was edited by JakobA the unAmerican. at April 22, 2018 1:25 PM MDT
The history of the alphabet is very long and hundreds of letters have been tried and discarded. For example, thee, thou, ye, and you all used to be the same word, spelled with a letter you probably have never seen.