If you try to generate a PGP key using GPG on a fresh Linux system, it may flatout refuse to do so claiming there is not sufficient entropy. Or at least it was like that some years ago.
As a result they added a "type into this buffer" to add entropy, which used what you typed and the timing of the keystrokes to help. Similarly graphical UIs say "draw in this region" for similar boost to randomness that helps avoid lack of entropy problems.