Big-Theta Notation (Θ-notation) provides a way to describe the exact bound on the growth rate of an algorithm. It gives both an upper and a lower bound, indicating that the algorithm's performance is tightly bounded within a specific range.

• Purpose: It shows that a function f(n) grows at the same rate as another function g(n) for large input sizes.
• Usefulness: While Big-O tells us the upper bound, Θ-notation provides a more accurate description by showing the algorithm's behavior in both the worst and best cases.