Taking an extra step
I see co-production as a quicker, lighter and affordable way to work on production aspects. If production means going all in, this is more about getting an extra hand. It often goes along with booking mixing sessions if, for example, there are sounds or parts that can be reworked to solidify a project's sonics. I am a big fan of going back to the production session to rework mixing problems from their source.
Collaboration as a catalyst
Booking this type of work is ideal if you want someone to bounce ideas off of, to get feedback while you're still in the process or to prevent mistakes that can later hinder a mix. It can be either production-focused (composing parts, synth patches,...) or creative-focused (creative direction, guidance and feedback).