Compressors make any bass or guitar tone instantly better by reducing transience and defining dynamics. What does this mean in normal language thought!? When we play amplified instruments a small over powered pluck of a string can translate into a "not very nice" higher note and likewise underpicked notes can leave the feeling of a missed beat. With the right settings the TLC comp will preventing any of this from occurring while maintaining the feel you're familiar with.
The TLC can also compress harder to provide tight unified bass lines found in metal, country and heavier rock styles.