To quote myself 15 years ago:
(N.B., “heel-toe” is a race driving technique for rev-match downshifting.)
…in ordinary street driving, in a car with synchronizers, there’s no real reason to heel-toe downshift between consecutive gears, or to downshift through all the gears.
On track (and conceivably on the street) heel-toe is used when you have to accomplish a large drop in speed, like at the end of a straight leading to a low-speed turn. You’re coming out of, say, fifth gear, and the corner calls for second gear, dropping from 130 mph to 40 in a couple of seconds. A lot of stuff is happening all at once, and if you were to let the clutch out in second gear just a little too early, you could over-rev the engine or cause rear wheelspin.
To be more specific for this OP, if you take your foot completely off the gas while braking from high speed in preparation for a downshift, the throttle will drop to near idle speed. The engine speed you will need for the lower gear will be considerably higher; the natural engine speed for the wheels’ speed may be 3,000 RPM or higher. If you simply release the clutch with the engine at, say, 1,000, the wheels will be trying to speed up the engine, which can cause them to lock up (i.e., wheel spin), possibly causing a skid, or over-rev the engine, which can damage it. Even if neither of those things happens, you’re putting serious and unnecessary strain on the clutch.
If instead you blip the throttle to slightly above the right engine speed for the current wheel speed and let the clutch out just as the revs drop to match wheel speed (rev-match), you can make a smooth transition from braking to acceleration without putting undue strain on the engine or clutch.
If you’re really good, you can slip into the lower gear without using the clutch at all. (Do not attempt! Professional driver on closed course.)