Fat is a concentrated energy source for animals just as it is for us. It greatly depends on your animal and their activity level whether they need extra fat. Activity can be thought of as not just athleticism, but also metabolic, such as keeping warm in the winter, or lactating. If there is not enough energy density in the other feed a rabbit might be able to eat as much of their ration that they can hold and still be losing weight, like you might if you ate nothing but celery and cucumbers and rice cakes--all you could eat without vomiting would not be enough, and you would still likely lose weight. Because I generally use a pelleted feed, I don't usually supplement, but in the past when I fed more scavenged greens I did feed BOSS, and for older does I have.
When we see a doe who can't keep weight on, she is usually outputting her consumed calories in the form of kits and milk, and not "saving enough" for herself. That is an example where we might want to up the fat in her diet. Some breeds may have growth spurts where they don't maintain weight and get skinny/boney and that is another time supplementation can help, as is cold weather. Other does won't get skinny, but may lose kits due to lack of milk production (they are "saving enough" for themselves at the cost of the kits). I think this is why BOSS is recommended for does specifically.
BOSS that is for birds is fine if it isn't moldy (bird feed is not always well regulated). Let the condition of the rabbit be the guide, and make changes slowly. If they are still looking skinny on 1 Tablespoon/day, up it to 1.5 or 2, if they start getting chubby, reduce.
I see Alaska Satin has just touched on this too! Sorry for the duplication, but since it is typed I will post it!