Almost every device with a multi-speed fan—fans, space heaters, air purifiers, the vent in the stove hood, etc.—has a switch that goes “off-high-medium-low.” The only exception I can think of is the A/C in my car which goes “off-low-medium-high.”
The portable fan in my bedroom has push buttons. The buttons go from left to right, Off, Low, Med, and High. So not all fans are Off, High, Med, Low. And being push button, I can start the fan at any speed.
(I mean it has four separate buttons, not one button you push several times.)
Because it is really not that simple. Anyone with a switched ceiling fan can tell you that it does not need to briefly start at the highest speed. It starts properly at any speed.
The “push” needed to start the fan is provided by a “start capacitor” which pushes the winding in a particular direction by causing a phase shift between the voltage and current. This happens regardless of the set speed (which is also usually set via capacitors).
I’d be surprised if it is to help turn the fan on. If you have a fan already set to low, you can plug it in and it will start just fine.
I think it’s more to make it evident when you’re turning the fan off. If it went off-low-med-high, then it would be easy to mistakenly switch from high to low when you really meant to turn it off. You would see the fan slowing down and assume it’s off, but really the switch is at low.
Gas ranges are like this. They go from off to high. I’m pretty sure that’s so you don’t leave it on low when you mean to turn it off.
When I was working in home automation, we used to take the wall-mounted fan controls apart and wire the capacitors to a series of relays in order to control the fans through an automated system. Whatever wall-mounted or touch screen device that we set up to control these relays had HIGH,MEDIUM AND LOW buttons that could be activated in any order.
There were a couple of different layouts for the capacitor arrays inside the controls. The HIGH speed was always a direct connection from power to fan. Sone of the 3 speed controls had two different sized capacitors- a small one for medium and a larger one for low. Some of them used equal sized capacitors and they were wired so the power ran through one capacitor for medium and both capacitors for low.
Here is a YouTube I saw just last week on this, answering in infinitely more detail. Heracles is essentially correct, it ensured enough oomph to get the fan moving from dead stop. Novelty Bubble is also correct, it’s not really necessary any more so it’s largely tradition.
This is the cheapest way to build fans and give the motor the needed push to get started easily. It isn’t required, there are many fans not built this way. But the cheapest ones are almost always built this way.