What is the purpose of adding curry leaves?
Most of your questions are a matter of personal taste!

1) What is the purpose of adding curry leaves?


  I'd say adding a nice taste to the meal (as long as you like it)


2) Does the taste of the curry change when you add curry leaves? Does it make it worse or make it better?


  yes it does. It will make your curry better... as long as you like the taste of it


3) Do any other country's recipes use curry leaves? If yes, which ones?


  I have seen curry leaves in many SE countries and cuisines.


4) Are there any health benefits of adding curry leaves?


  They are believed to have multiple benefits 1, 2, etc. 


5) Is it better not to add curry leaves?


  Well, again, as long as you like it


6) Are curry leaves added to any other non-curry recipes?)


  I have seen them in many other meals than curry both in India and in SE Asia. Here is a primer for some recipes using them.