I have observed INnoVation rallying his production buildings to a Supply Depot outside his natural for long a time. I have also seen others doing this, but it is definitely not common. Although I have a rough idea of how it works, I decided to post it on reddit after I have seen him doing that on his stream.
As everyone knows, Terran production buildings are always rally to a specific point. This rally point varies according to match up and situation. More often than not, the rally point is set near the base. INnoVation places his rally point on a Supply Depot outside his natural. There is nothing wrong or unique to rally outside your natural, but why on a Supply Depot?
That Supply Depot is kinda “out of place”, as it does not really serve any specific purpose. If he wanted to create choke points, more Supply Depots would be required anyway. After watching his stream (first player view), I conclude that the Supply Depot is built there to serve as a rally point. He often “cut” his screen to the rally point to select his reinforcement. Then, why not just rally to the ground anyway? I thought he may have created a control group on the Supply Depot, and this helps him to quickly move to his unselected reinforcement. However, you can also create a location hotkey, which serves the same purpose. In fact, location hotkey may be better than a “hotkey-ed Supply Depot”. You can be more flexible with your location hotkey as it can be set anywhere, and is not restricted to the built Supply Depot location.
Unspokenowl on reddit pointed out an interesting point.
Usually this is so that they do not just rally pass an army. If a supply depot is destroyed that the front of the base, I am pretty sure this resets the rally. I could be wrong though, but I believe that is accurate. So if the enemy storms the front and takes out the depot, his army wont keep rallying to their death.
It does have its merits.
I have also noticed that INnoVation consistently rally to a lowered Supply Depot at the wall of the natural in his TvZ games. Basically, in TvZ, you make a wall at the natural with mainly Supply Depots. One is always lowered to allow reinforcement to walk out of the base. A lowered Supply Depot also means that the base is vulnerable to Zergling run-by. By rallying on the lowered Supply Depot, the units create a wall at the entrance that prevents Zergling run-by. The units are also in the best position to kill those Zerglings.
INnoVation does not record past broadcast. So I cannot verify if he used location hotkey or control group on the Supply Depot. Nevertheless, this seems to be a good detail to implement into my own games.