Why is ordinary time an important part to the liturgical year?

Because Ordinary Time is when the Scriptures are read sequentially, in Order. Which is why it's called "Ordinary." The Word in this context doesn't mean "not special." It means "Time that's counted."

In Year A Matthew is read through. Year B, Mark; Year C, Luke.

Ordinary Time includes all those weeks that do not fall into the seasons of Advent, Christmas, Lent or Easter.