To bring the source up to a recordable level. Some preamps add color, character, texture, brightness, darkness, or punch. Some preamps add hiss, noise, nasality, tintinabulative qualities and make things worse. Some make dreams a reality, some make reality a nightmare.
A lot of good microphones require the boost that a preamp offers.
Now do a search and learn from some really smart technicians
In His Name
Big Kenny