Poly count is used to difine how many polys in your model. Its useful to count because if you for example create a game character, you often have to keep your number of polys under a certain amount (guess it bacause of system requirements etc.).

Also, it tells how complex your model is. More complex models have longer rendering time.

You can turn on poly count by selecting Display > Heads Up Display > Poly Count.

Maybe someone can better explain the use of poly count... ?