Yeah, I find no real reason to do it in daily driving other than being bored, or perhaps if your synchros have suddenly gone to shit (pairing it with a double-(de)clutch in that case. Actually, when I am bored, I do try to double clutch anyway, just to, I dunno, just for fun I guess. I suppose it’s because when I was first taught heel-and-toeing, it was in conjunction with a double-declutch.)
And, yeah, “heel-and-toe” is a bit of a misnomer these days with the pedal layout of most cars. It’s more like ball-of-foot and side-of-foot on all the cars I’ve owned. The term comes from when the accelerator was in the center, and the brake was on the right, and the heel would work the brake, and the toe the throttle (this was mentioned before either in this thread or the thread that inspired this one.)
I prefer to ride a motorcycle, and it’s a very common low-speed technique to use throttle, clutch, and rear brake at the same time.
The throttle keeps the engine speed up, and the clutch and brake decide how much the rear tire is actually moving without stalling (and dropping) or lurching.
Also, in years past I’ve been forced to drive manuals in the left foot clutch only, right foot simultaneously actuating the gas/brakes, similar to race car methodology mentioned upthread. The difference being: I was nursing poorly a tuned VW in stop and go traffic… it wouldn’t idle, and the engine would die if I didn’t constantly rev the thing. In this kind of situation one gets pretty handy in the technique of gas/brake/clutch operation with only two feet all at the same time, not because of any race driver fantasy, but because it was necessary.
Back to the OP: For the life of me however, I can’t envision why an experienced driver, operating a garden-variety properly running manual transmission street vehicle in “normal” circumstances, would even consider routine braking with the left foot. I offer no cite, but the ergonomics of the situation are self explanatory… similar to why (in the USA) we shift with our right hand.