Why do they put a diode in parallel with the relay coil. What is it for?

In the diagrams, in which the relay coil is installed in the load on the collector or drain of the transistor, you can see that a diode is installed parallel to the coil relay, with the cathode to the positive power supply. With this connection of the diode, the current through it will not go to the transistor. Then what is it for?

This diode is necessary to bypass the relay when the power is turned off, when the transistor opens. When the power is turned off, an EMF pulse (electromotive force of self-induction of the coil) is formed at the coil terminals, the voltage of which can reach tens of volts, which can lead to failure of the transistor, which is not designed for such voltage. These pulses can simply disrupt the operation of the device. The diode, closing, bypasses the relay, since at the moment of the EMF pulse, the polarity changes at the coil terminals and the diode closes, that is, minus EMF appears on the cathode, and plus on the anode.

These self-induction pulses can be seen on an oscilloscope. I assembled a simple circuit consisting of a 3.7V power supply, a button, and a relay coil with a diode. I connected the oscilloscope probe to the coil terminals.

First check without diode, if there is no diode installed in parallel with the relay coil. You will see that individual pulses can reach 82 volts.

Now I connected the diode and the pulses disappeared. This means that the diode works in this circuit, the main thing is that the diode is designed for such voltage.If the transistor can withstand high voltage, then the diode probably doesn't need to be installed.