Almost every studio you can find a mixer and any audio interface you will find a preamp built-in. In most cases, the preamp will be relatively low quality compared to promote a separate external amplifier. Not all pre-amplifier suitable for every task, each quality sound and a different hue.
Why use an external preamp? Mainly because of the quality. Quality preamp, comes as a separate unit created solely for a specific purpose and not as part of a larger whole usually hear much better, shall be capable of high-frequency response, with sound clean and clear. Price, will be in accordance with the course. No matter what technology works pre amp and although each of them produces a different sound, they all do the same job in the end. The pre amps dominated the music industry in the sixties and seventies did not produce more. They all claim they just sounded so much better. To date, even the most innovative equipment, still could not reproduce the same sound wonderful. Which is one reason they have a great demand.
Why did they sound so different?
Probably the real reason is the components inside. Converters and inductors for example routinely used at that time, no longer useful today because of the relatively enormous size, weight and price. So why just do not make them like before? Few big companies and selling in trying, without success. Elements of the ancient amplifiers them just do not exist, and certainly not the companies that produced them. In addition, some components manually built and designed by experts and it seems no one is able to recover. There are a lot of innovative amplifiers good and even very good ones who do their work faithfully as a member of the Mackie XDR or Vt737sp Avalon eg company. Others focusing on trying imitate the sound of their predecessors and some familiar production focuses on clean and high quality sound.
Whether you manage to get a classic preamp second hand or from a broken mixer, and if you purchase one of the newer models, you should choose an external preamp and higher quality if your recording level is really important if your financial capability allows course.