I came across this Theorem in Introduction to Mathematical Statistics and I don't understand what it is stating:
Let ${C_n}$ be a nondecreasing sequence of events. Then $$\lim_{n\to\infty} P(C_n) = P(\lim_{n\to\infty} C_n) = P(\bigcup\limits_{n=1}^\infty C_n).$$
P is a probability function here. I understand the statement that $\lim_{n\to\infty} C_n = \bigcup\limits_{n=1}^\infty C_n$ for a nondecreasing sequence of events, but that is a result of set theory. What is the significance of interchanging P and the limit in the theorem above?