The real reasons for left foot braking are many and its advantages obvious to those who are good at it.
Besides the faster reactions mentioned, there is also the reason of finesse. It allows you greater control of driving forces than any other technique. In rallying that uses active diffs it actually allows one to choose between different differential settings in real time. It also allows real time manipulation of effective brake bias, provides better control for trail braking by superior control of throttle/brake overlap, etc.
For top level SSs such as F-1, left foot braking is an absolute must on the skills mastery list to have any real chance of competitive racing. This is actually why people like Micheal Schumacher and Aryton Senna were such amazing F-1 drivers, their left foot braking skills allowing for unmatched finesse. And no, heal and toe is NOT equivalent to left foot braking. A single foot contorted to manipulate pedals a once will never be a match for 1 foot per pedal. In fact, many people simply fail to master F-1 and rallycars simply because they just can't get around the idea of left foot braking. If anything it makes heal and toe look relatively easy.
With heal and toe, you just need to get used to multitasking and pressing 2 pedals with only your right foot. This is actually very impractical or virtually impossible to do with a lot of cars, especially with vehicles that have TINY brake and throttle pedals that are spaced a mile away. Can be done in many circumstances, but not without tendon and ligament torturing contortions that will cause permanent damage if practice over a significant length of time, especially for people with feet that requires smaller than size 10 shoes. The good news is that these days there are plenty of replacement pedals/pedal surfaces that improve pedal size and surface area and allow extended heal and toeing without the associated RTS (repetitive task strain).
With left foot braking the learning curve is actually much steeper as it requires a level of pedal finesse most can never master. It SEEMS simple in theory but is actually VERY difficult, especially for beginners. The required mastery of left foot braking is one reason why F-1 cars should remain pedal shifted, since it allows those who master this difficult skill to shine. How one has to stomp on the brake with the left foot in these VERY HIGH downforce cars and then relieve pressure on the brake pedal as speed and downforce decline whilst adding throttle to move effective brake bias to the front. If it sounds even harder than heal and toe, that's because it is.