A buffer has a propagation delay time from input to output. If a customer can not live with that delay time, they choose the Zero Delay Buffer, hence the naming. ZDBs has the ability to be able to align all output edges to the REF or even leading the REF depending on the delay of the output to feedback path. If the source is clean and propagation delay is fine, non-zero delay buffer will provide less additive jitter. If the source has jitter and needs filtering of the same, ZDB becomes the obvious choice.