Plans are intended more for day-to-day collaboration around the next steps/MAPs. They organize the process and hold everyone accountable. Plans are also where discussions can happen in context, which makes things easier for the buying teams vs many email threads that can fail at organizing the deal.

The process overview (timeline) helps give a high-level overview of major milestones up to value delivery. They are also collaborative. You can change it with a prospect during a call, or use comments to align on changes in the overall process a-synchronically. However, different from Plans, the key goal here is to create urgency and give better visibility into what the overall process looks like and less ongoing day-to-day collaboration.