Applying stereo widening to reverb/delay returns keeps its side effects off the dry signal

You can apply stereo-widening tactics to reverb and delay return channels rather than to dry tracks. The advantage is that the widener’s timbral and mono-incompatibility side effects cannot afflict the dry signal directly, so vocals and acoustic instruments stay natural, and summing to mono weakens only the effect — often no bad thing in dense arrangements where mono clarity is hard to keep.

Examples

A hall return processed with a mid-side widener extends further to the sides than the dry tracks; in mono only the effect narrows while the vocal stays focused and natural.

Assessment

Explain the advantage of widening a reverb/delay return rather than the dry signal, referencing mono compatibility.