These facts is all about it your question :

1- In fact, that diamond used in machining operations is "Not" natural diamond , it's just a synthetic artificial diamond called :

2- It's advantages :

*Highest hardness >> resist wear >> long life for the tool
*Chemically stable >> resist corrosion and
*capable to cut in high speeds,
*produce a good surface finish product
*excellent dimensional accuracy and
*heat resistant >> withstand cutting at very high temperatures

3- disadvantage :

* High price
* brittle >> does not withstand shocks and heavy loadings
* Limited to nonferrous materials only.

Now it should be clear why we use synthetic diamond in cutting : because of its many advantages and it high productivity.

Hope this helps , if you have any other questions just messege, it's my pleasure to answer.

Good Luck