builtin command makes sure you run the shell built-in version of the command rather than running another command with the same name.
For example, let's say you defined a shell function named
cd to print some extra status everytime you change directories. But you messed it up and now you can't change directories correctly. So now you can type
builtin cd ~ to successfully cd back to your home directory without running your broken shell function.
And by the way, my copy of the bash man page has a section called "SHELL BUILTIN COMMANDS", and it defines the meaning of the
builtin command in that section (transcribed below).
builtin shell-builtin [arguments]
Execute the specified shell builtin, passing it arguments, and
return its exit status. This is useful when defining a function
whose name is the same as a shell builtin, retaining the
functionality of the builtin within the function. The cd builtin is
commonly redefined this way.
The return status is false if shell-builtin is not a shell builtin