there is a distinction between reverb - a wash of reflections that overlap each other - and echo which are discrete repetitions of the original sound.
You design your room/booth to eliminate both, or to enhance one (usually reverb by using ceramic tiling on the walls). The main design considerations in the design of a vocal booth/room are the elimination of standing waves - resonant points for a specific frequency - rather than the elimination of reverberation specifically, it generally comes as an add on in fighting the the first battle (standing waves).
So adding some reverb to vocals is intended to compensate for what couldn't be done at the recording stage.
As an aside, having the ability to precisely control how much reverb is on a vocal is a good thing, much easier than trying to tame a poorly designed room or a weak vocalist to reverb ratio in the mic signal