The brilliant iridescent throat feathers on the male hummingbird serve two very important functions: attracting a female and defending their territory. They use these to display to one another. Females find it attractive, and other males are repelled. Male hummingbirds are extremely feisty and territorial and these feathers are part of their territorial display.
It may surprise you to lean, as it did me, that in the mid-1800’s hundreds of thousands of hummingbirds were killed for their feathers which brought many species to the brink of extinction. Luckily, this did not happen. I myself am very thankful for this because, in my opinion, the world would be a far less beautiful place without the extraordinary and beautiful hummingbird in it.