[c22]

I know because castling developed from the King's Leap [0] which came about as the game evolved to faster more exciting play. By allowing the Queen and Bishops to move an arbitrary number of spaces various mechanisms were tried for protecting the King revolving around moving multiple spaces or jumping pieces. A common sequence became to move one's Rook adjacent to one's King, then using the next move to "jump" the King to safety. This eventually became formalized as modern castling and can be seen by noticing how the pieces move when castling kingside vs queenside.

[0] http://history.chess.free.fr/mediaeval-chess.htm