In which tasks is a stack most often used?

A stack is most often used in tasks where you need to go back or process data in reverse order. The main examples:

1. Recursion and function calls: storing arguments, local variables, and return addresses (the system call stack).
2. Checking bracket validity: convenient for tracking opening and closing brackets in expressions.
3. Reverse Polish notation (postfix form): when evaluating arithmetic expressions.
4. Undoing actions (undo): storing the history of changes in text editors, graphics editors, and similar.
5. Traversing graphs and trees (DFS): a stack is used to store the vertices that still need to be visited.
6. Parsing and compiling code: tracking nested constructs.

In all these cases, being able to "go back" to a previous state matters, and that is what makes a stack an ideal tool.