The buffer ‘*scratch*’, which is selected when Emacs starts up, is provided for evaluating Lisp expressions interactively inside Emacs. Both the expressions you evaluate and their output goes in the buffer.
The ‘*scratch*’ buffer’s major mode is Lisp Interaction mode, which
is the same as Emacs-Lisp mode except for one command, LFD. In
Emacs-Lisp mode, LFD is an indentation command. In Lisp
Interaction mode, LFD is bound to
function reads the Lisp expression before point, evaluates it, and inserts
the value in printed representation before point.
The way to use the ‘*scratch*’ buffer is to insert Lisp expressions at the end, ending each one with LFD so that it will be evaluated. The result is a complete typescript of the expressions you have evaluated and their values.
The rationale for this feature is that Emacs must have a buffer when it starts up, but that buffer is not useful for editing files since a new buffer is made for every file that you visit. The Lisp interpreter typescript is the most useful thing I can think of for the initial buffer to do. M-x lisp-interaction-mode will put any buffer in Lisp Interaction mode.