Producers layer sounds when no single source has everything they want. A kick with deep low end but little attack might be paired with a clickier sample; a snare might gain a brush or rim sound on top. The layers must be lined up carefully, because misaligned transients smear into a flam, and opposite polarity at the start of two sounds can cancel frequencies and thin the result. A common guideline is to let one layer own the lowest frequencies.
In jazz lo-fi, layering also means building atmosphere: a sampled chord loop, a played bassline, soft drums, vinyl crackle and a field recording of rain stacked into one warm texture. Shared processing, such as compression or light saturation across the group, helps the layers sound like a single performance.