why is the negative feedback loop important for maintaining homeostasis?

A negative feedback tells the body to stop producing a certain product (perhaps a anabolite or hormone.)

For example,

When your pituitary gland releases a hormone. It continues to release the hormone until it receives negative feedback from the body to stop releasing the hormone. This is done so that too much of the hormone is not released; thus maintaining homeostasis.
 
The idea of negative feedback can also be applied to the production of repressible enzymes in anabolic pathways.

In a anabolic pathway your bodys machinery synthesizes a product. It continues to synthesize when the product is scarce. Stops when the product is abundant. (Like in transcription of certain enzymes)

This is another example of negative feedback.