Slapback Delay is used a lot on vocals and guitar.It can fatten up the sound of a vocal or guitar track.It is especially useful on guitar for Blues and Chickin Pickin,and Jeff Beck uses slapback delay with a heavy mix,on a lot of his playing.
If,say,you can not build a delay preset or patch to match the tempo of the song,slapback also works well as a substitute.This is the problem with a lot of pedals without a "tap" delay function.You can not set up different delay patches or presets,that have different delay tempos to match the song you are playing!
Slapback delay usually falls between a timing of 40 to 120 milliseconds.So it is very quick,and does not require to be "in time" with the song tempo,while at the same time,it fills the purpose of "fattening" up the sound!
Some audio examples of delay times and slapback delay...
http://www.fender.com/products/dynatouchIII/home.php ..."Chickin Pickin"