Hi Ah Keong
I use Zone AF on the 7DMark 2 when I need to find a fast-moving subject quickly, whilst it is still small in the frame (and I might miss with Single Pt), situations might be an approaching bird in a clear sky that i want to start tracking from far off. I may shift the focus mode from AF Zone to a Single Point or Single plus 4 (AF expansion) immediately after aquiring initial lock, if the bird is far enough away to give me that time. It is easy to switch on the 7d2 with that mode lever. Other times i use Zone AF on the 7D Mark 2 are when my subject is moving from one side of the frame to the other, rapidly. The advantage of Zone AF is that i can move it from left to right of the frame with just one or two clicks of the multi-controller.
Sometimes I find that Zone AF grabs the wrong part of my subject though, like the wingtip or the tail, but this might not be an issue when I am shooting with sufficient depth of field. I have spent enough time shooting the subjects I most often shoot with the 7d Mk 2 that I have a feel for how it does for me. It may just be me and the subjects that i shoot, lenses etc but i typically find that AF Zone works a tiny bit faster and more accurately on the 7D Mk2 than on my 5Dmk3 and perhaps my 1DX as well.
On the 5Dmk4, I am still evaluating my own results and building experience with the focusing modes, and so far I am getting my best results with Single Pt Af, as well as AF Point Expanded (1 plus 4) and then for instances where I may need to find a bird in the sky, exactly as described above for the 7d2, I am using AF Expansion (one plus 8 points) to give me wide coverage, at least initially. I may end up using AF Zone on the 5Dmk4 more when I have more months with the camera?
It feels like each new generation of processor or camera that Canon bring out in the 7D, 1DX and 5D series get a little better at managing the more complex AF groupings so I am cautious not to assume that one body will be the same as another without shooting a lot with them first. Some models also have more powerful dedicated AF processors