BBF is benefit based on several factors, such as subject, environment and how one likes to use a camera, or how much control one wants to give to the tech as much as anything. Like Bill (Waddizzle), I shoot exclusively with BBF, and in fact I set the * button to its right to allow me to lock AF as well, and I have done so for many years.
Most cameras these days are reasonably good at guessing focus and exposure, but they are by no means foolproof and will sometimes select values or locations that work for the algorithm but not for the shooter. FWIW, here is how I set my camera up and why.
I have focus disabled from the shutter button, and BBF is already set by default for most Canon Cameras. Since I shoot with several different makes and models of camera, the details vary a bit and that is also why I chose a focusing and exposure system that works across them all.
Essentially, I shoot in single point focus with servo and eye tracking. If the subject is static, I place my focus point on the eye of the subject, prod the BBF button and it locks focus. If the subject is moving, and if the camera model does not have eye tracking, I hold down the BBF button to track. Some models do not require that process. The reason I have set up single point specifically is to be able to isolate the eye of the subject. That is particularly relevant in deep forest of bush when I am working with birds that are small, flit a lot, and don't come out into the open much. Just having eye tracking alone would decrease my control as a focusing system often gets distracted by leaves and branches. One of the features I use is the assignment of the centre button of the rear control wheel to centre the focus point. So, if for some reason I lose track or the camera finds it hard to get a lock in dim conditions, I can just centre the eye on the screen, lock the focus point and then it will usually track. My R5 and R6 units have the benefit of different eye/head tracking modes, and I have assigned each with a specific set of other parameters to each of the C1-3 mode values for fast configuration. This is less of an issue with the R6II, as its new Auto feature is reasonable good at figuring out what to track and holding that.
I choose to use single-point exposure, assigned to the * button, to allow me to determine what I consider has the closest to mid-tone value in an image. Again, using a single point gives me precision. In operation, when one gets used to it, the process of acquiring and locking both takes very little time, as the two buttons are adjacent, and I get a fairly high keeper rate.
In the example below, of an Elegant Green Gecko (AKA Auckland Green Gecko), an animal that is barely 125mm (5") from nose to tail, it was taken in extremely dim light, and the DoF is wafer thin, with lots of distracting elements as the animal's camouflage merges so well with its surroundings.
In this case the gannet was tracking along at a fair pace with the seaweed flapping in its eye (causing a fairly spectacular crash landing later).
