Let's say, I have a custom function to run some operations using the details of a record. Now I want to run the same function on 100 different (and independent records) and each run of the function takes 5 mins.
Essentially each of these are independent function runs and can execute parallelly, hence requiring a total of only 5 mins. But since these are executed sequentially, this takes a total of 500 mins.
Simply speaking, parallel execution leads to 99% reduction in time and is really helpful where updates need to happen frequently. I can create separate tasks for them which trigger at the same time but managing 100 different tasks is a huge pain. Hence, parallel execution of functions, or the ability to trigger functions at a certain date/time such that many of those can be executed at the same time would allow significant performance improvements for apps where frequent updates are required.