Gorse, as I understand it, was brought to New Zealand to be used as fencing between farm fields.

This seems likely since it was used for that back in the UK. Also the flowers were used for making dye.

I understood it was also because of the pretty yellow flowers and that it didn't grow so well in western Europe

It grows plenty well in western Europe.

I was quite taken by the hillsides that were completely covered in bushes with bright yellow flowers. I asked what they were, and was surprised to find out they were an invasive species. Sure were beautiful though!