HIP is not merely a "historically informed" way of performing familiar music, but a way of rediscovering works that have been lost or neglected. Along with the resurgence of obsolete instruments and styles, entire repetoires get unearthed, works that would have remained buried if not for historically informed practice. For instance, Marin Marais's pieces de viol were written specifically for the obsolete viola da gamba, and they can't satisfactorily be performed on any modern instrument. Listen to Jordi Savall, who knows a thing or two about historically informed performance: "[The] Countess of Chambure [loaned me] one of the bass viols [i.e., a viola da gamba] in her private collection. Suddenly, all the nuances on which I had been working so diligently, such as 'bowing on the air,' became clear to me as the suppleness and sensitivity of that seven-string [and please note the SEVEN strings] bass viol made the thousand nuances that Marais's music required both natural and possible."
You can't adequately play Marais's music on a modern (i.e., four string) cello, although it is true that Marais himself allowed substitutions. It can't even be claimed that the cello evolved from the bass viol, as not only were they around at the same time, but the violoncello was called the "basso di viola da BRACCIO." Hence, even though it is played between the legs, it was classified with those viols that are played on the arm (braccio) in contrast to the viola da gamba (that are played on or between the legs). So the bass viol was another beast entirely from the cello. To hear what ONLY historically informed practice can achieve, pop "Marin Marais: Suite d'un Goût Etranger" into your SACD player.
To perform what is to be heard on these discs, you NEED a period instrument and a lot of delving into the past. If not for historically informed practice, this repertoire (some of the most beautiful music ever written) would not be available today. So HIP not only presents us with a certain way of playing music we already know, but brings us music that had previously remained unheard. It not only recovers lost playing styles and instruments, but musical works as well.