I don't understand.
On the one hand, you are saying that there is no 'z' sound in the script.
But then you say that the dot denotes 'z' rather than 'j'.
So, there is a 'z' sound, but people tend to gloss over the dot and pronouce the 'z' as 'j', is that it?
Its more of an addition made to include the "z" sound in Hindi. As far as my knowledge goes, it wasn't inherent in Hindi Devnagri script. Hope this helps.
Like the scripts of all languages, the Devnagari script too works with a finite set of sounds, and so many sounds that occur in foreign languages cannot be represented in Devanagari. Two common sounds of English that cannot be represented in Hindi are the "z" sound as in zoo, dizzy, etc., and the "o" sound as in doctor. The closest sound in Hindi for "z" is the consonant ज(j), which is often used to indicate the "z" sound. Sometimes a dot is added below it (ज़ to distinguish it from the normal ज(j) sound. For the "o" sound a new vowel has recently been added to the alphabet of Devanagari, which looks like this - ऑ. However, these symbols are as yet not universally understood and should be sparingly used. The recommended practice is to adjust the pronunciation of foreign sounds to fit into the normal set of sounds available in the Devanagari script. So instead of writing ऑस्ट्रिय (Austria) or ज़ांबिया (Zambia) one should write आस्ट्रिया or जांबिया.
English to Hindi Translation