When you want to run more than one of the same methods with task processing

If you don't have to consider anything, it's simple to run multiple tasks in one instance.

It remains to be seen whether such tasks alone can produce practical applications.(Well, I don't think that's necessary.)

How to use the variables in the class

This is usually the most important decision point, and you decide whether or not to use one instance by checking whether the "instance member" is "thread safe" in references such as MSDN.