Why do we define topology like this?

Question

In the book "Topology" by James Munkres we define topology on a set $X$ like the following(page 76):

$\mathscr T$ is a topology on $X$ if:

• $\emptyset,X\in\mathscr T$
• $T\subset\mathscr T\implies\bigcup T\in\mathscr T$
• $T\subset\mathscr T\land T\mbox{ is finite}\implies \bigcap T\in\mathscr T$

Why does the third property have the fact that $T$ is finite? What does it gives us?