A basic chorus effect
The "chorus" effect is commonly used to enrich a sound. The effect gets its name from the way that a chorus of people singing or speaking in unison sounds different from a single person. By extension, a group of violins sounds different from a single violin (even though we don't call a violin section a "chorus"), and the same effect even takes place inside a piano because most of the hammers strike two or three strings tuned in unison rather than a single string.