This bit from Ableton's Learn Live series shows how nesting an Instrument Rack inside another Rack unlocks a workflow most producers never touch. Put a MIDI effect like Scale ahead of your instruments and an Audio effect like Hybrid Reverb after them, all within one nested structure.
The payoff is unified control: all 16 Macro knobs on the outer rack can drive MIDI effect parameters, instrument parameters, and audio effect parameters simultaneously, so a single sound design idea, like tightening a scale while opening up reverb, plays out across the whole chain from one interface.
Nesting doesn't break your existing Macro mappings either. Map the original rack's Macros to the new outer rack's Macros and everything carries over, so you can build up increasingly complex instrument designs without redoing your control setup each time.