[1][2][3][4] Shell builtins work significantly faster than external programs, because there is no program loading overhead.
Therefore, shell builtins are usually used for simple, almost trivial, functions, such as text output.
Because of the nature of some operating systems, some functions of the systems must necessarily be implemented as shell builtins.
The most notable example is the cd command, which changes the working directory of the shell.
Since each executable program runs in a separate process, and working directories are specific to each process, loading cd as an external program would not affect the working directory of the shell that loaded it.