https://cdn.modulargrid.net/img/racks/modulargrid_1105457.jpg
Left Hand Side or Right Hand Side??? The Left is probably a load cheaper!!
The ability to come out in stereo with some final effects and maybe have the ability to set up some effects loops (Feedback loops) to come back into the mix.
Enjoy your spare HP, don't rush to fill every last space, this is not like filling sticker books. Resist the urge to 'complete' your rack, its never complete so just relax.