long explanation is that a brisket gets to be tender by breaking down the connective tissues between the muscle fibers. This breakdown is a function of time at temperature, or temperature over time. To illustrate, let's imagine that you could instantly raise the internal temp of a brisket to exactly 200 degrees and could hold it there. It might take an hour for the tissues to render at that temp. IF you did it at 190, it might take 2.5 hours. At 180, it might take 4 hours. Remember, I'm not talking chamber temp, but rather, internal temp.
Of course, when you throw a brisket into your smoker, the internal temp doesn't instantly jump that high. It will be sub 40 degrees when it first goes in and will slowly inch upwards one degree at a time. For sake of simplicity, let's look at 10 degree increments. It might spend 1/2 hour at between 40 - 50, another 1/2 hour between 50 and 60, then next 1/2 hour might see it accelerate up to 80, another 1/2 hour it's up 100, etc, etc, etc. As the internal temperature increases, the rate at which the connective tissues break down accelerates. This continues right up until the stall. Once the evaporative cooling has finished, the internal temp starts to rise again, but this time the rate of increase slows down if cooking at a lower chamber temp. This is because you are close to reaching an equilibrium point as the internal temp is getting closer and closer to the chamber temp. At a higher chamber temp, the rate of temp increase will be faster.
All that is the long way of saying that the tissues break down (and the brisket gets tender) faster when cooking at higher chamber temps. And yes, the fat renders nicely as well.
A kind of inverse illustration would be to picture melting a block of ice. If you put it in a 40 degree fridge, it will take a long time to melt. If you put it on the counter at 70 degrees, it will melt quicker. If you put it in a pan and in the oven at 350, it will melt a lot faster.