1. How and where it should be displayed?
It can be displayed alongside the task name e.g.
* TODO My repeated task (DONE=3)...
Here DONE=3 meaning the task has been completed 3 times.
Another way to display is: DONE=3/4, where it means the task is completed 3 times out of 4.
But it is much more complicated because the user can change the repeat duration.
Another thing is to show the longest streak and last streak count, but these are also complicated I believe based on the information that is logged.
2. When it is going to be useful.
It is very motivating to see how many times you have complicated a specific task. It is much like a streak. People would wanna see their statistics to put more effort. I strongly believe "The best way to get better at something is to measure it".