they bring mic signal (LoZ) level up to line level.
They will also color the sound and effect single transparency.
SM57s are dynamic mics (don't require phantom power) and not very responsive. They will sound very different to a small or large diaphragm condenser, which do require phantom power.
However that said, even a really cheap mic pre / channel strip will sound better that the average mic socket on a sound card.
Don't plug a mic into a guitar (HiZ) or a line level input as it will sound horrible.
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