I think it's something similar to the flyback diode for protection from reverse current. And according to symbols, there is Schottky diode.
Hello there and welcome here.
That's a very interesting reply. First, you asked what the diode was for, then I told you want it was for, then you said you still think it is for some other reason. It appears that you were not prepared to accept any answers.
Myself and many others have worked with this circuits for years and years, so it is more than likely that we know what that diode is for. I realize though that these circuits are a lot more complicated than the simpler linear type regulators, so it's going to take more effort to understand them.
When the transistor internal to the IC chip turns on, it pumps current through the inductor. Some of that current charges the capacitor, some goes right to the load. That puts some voltage at the output to power the load.
When the internal transistor turns off, the inductor wants to keep pumping that current to the output. To do that, the voltage across the inductor flips so that the left side becomes negative. To keep the current flowing, the diode allows the current to keep flowing from ground through the inductor to the output. That helps to maintain the output voltage and keep it somewhat smoothed out.
This also partially 'resets' the inductor getting it ready for the next 'on' period of the internal transistor. Once the transistor turns on again, the cycle repeats.
If the diode was not there the inductor would not have a discharge path when the transistor turns off, so the circuit would not be able to function properly.
You can simulate this in a circuit simulator. There is a free one they call "LT Spice" but I think the actual name is "Switchercad". It was made by Linear Tech and you can download and use it for free.