Red Polls feature on the rare breeds listing (https://rarebreedstrust.com.au/public/pages/cattle) because there are less than 250 cows. This figure must come from site that only refers to registered animals, as there are more than 250 in Australia as far as I can tell.

Why do rare breeds need preserving?

In short it is because we need to preserve genetic diversity in all our food species. Of course, there is the obvious reason in the case of Red Polls that they are high performing, excellent cattle that easily hold their own against more fashionable, but less excellent breeds (*cough angus cough*). There is a great article on this by Dr Catie Gressier in the ARPCBI 2022 Annual. Have a read if you are interested in the Value of Breed Diversity.