Shellcode

Shellcoding is an excellent way to learn more about assembly language and how a program communicates with the underlying OS.

Why are we red teamers and penetration testers writing shellcode?

Because in real cases shellcode can be a code that is injected into a running program to make it do something it was not made to do, for example buffer overflow attacks. So shellcode is generally can be used as the “payload” of an exploit.