Here's why:

* Cathode: The cathode is a negatively charged electrode in the CRT. It's typically made of a metal with a low work function, like tungsten or barium oxide.

* Heating: The cathode is heated to a high temperature. This heat provides the energy needed for the electrons to overcome the work function of the metal.

* Emission: When the cathode is heated, electrons gain enough energy to escape the surface of the metal and become free electrons in the vacuum of the tube. These free electrons form the electron beam, which is then accelerated towards the anode.

So, the process of thermionic emission is crucial in generating the electron beam within a CRT.