If not set properly they can be worse than the noise. A delay by itself has no effect but to delay the signal. When the delay is heard mixed with the original signal, we have a sometimes more interesting sound. These signals mix together to form artificial reverberation as found in different size enclosed spaces. This fattens up vocals and instruments and can make it sound like there was more than one instrument playing the same part.
Short delays can also add fake ambiance to a recording that was too dead sounding.
Chorusing is caused by modulating the delay time. This modulation causes a change in pitch as well as a change in the delay time. This produces a wavy effect in the sound. Flanging effects are created by using a delay of 10 to 20 milliseconds and changing the delay amount slowly between those two parameters. The delayed signal mixes with the original signal and some of the frequencies are out of phase with each other and cancel or augment each other.