Preamplifiers came into existence to equalize and amplify the very low-level phono signals from a turntable. Most tuners and tape decks had their own volume controls, and didn't need a preamplifier when used on their own. Also, many power amplifiers had their own volume control. After CD players became popular, and turntables became relatively rare, preamplifiers became (theoretically) "obsolete" and/or "redundant".
However, rather than lose the sale of a (now) obsolete component, the manufacturers began marketing a "new" component, the "line-stage" (which is just a preamplifier without a phono stage).
Nowadays, preamplifiers are used with "seperates" systems for sound format decoding into various channels to feed power amplifiers.
* Explanation courtesy of Arthur Salvatore at High End Audio Inc.