Paul Womack explains why a 16th-note slap delay often works better than reverb on rap vocals, and how he shapes the effect to fit the specific voice he's mixing.
The core idea is that reverb implies space by filling it, while a slap delay implies space without occupying it. For staccato, rapid-fire rap delivery, long reverb tails blur the words and weigh down the performance. A tight 16th-note slap gives the sense of a room without competing with the vocal.
The less obvious move here is EQing the delay return rather than leaving it flat. Womack ducks around 2kHz on the effect to avoid amplifying the vocalist's natural nasal character, and rolls off the low end to keep stray low frequencies out of the stereo width. The delay becomes something you feel rather than notice.
This approach applies any time a vocalist's tone has a quality you don't want reinforced by the space around them. The effect should complement the voice, not magnify its problem areas.