what is the importance of the nitrogen cycle?

Well, what's important in every cycle is that everyone/user in the cycle gets its share and functions to change the form of the object for another user's use.

In the case of nitrogen, it is useless to almost all lifeforms in its pure form. That's why specialised bacteria (I think they're called soil bacteria) takes its part in converting nitrogen so that it can be used by plants. Then animals eat it and obtain the nitrogen that was taken in by the plant and when they excrete, their excretion will go back into the soil where other types of soil bacteria will convert it back to its gaseous form.

Decomposers break down dead organisms into nitrogen into the soil.