Despite all the hype, the recent launch, several PPS related conferences, numerous articles and the pull-no-punches Olap Report Preview ,there is still some doubt about what the Planning element actually offers, well..
To quote directly from the marketing bumf:
Efficiently build budgets, forecasts, and plans in the interface everyone knows—Microsoft Office Excel. PerformancePoint Server 2007 offers auditing capability, centralized control, enhanced security, and the proven data platform of Microsoft SQL Server 2005
For a specific example, imagine, without PerformancePoint Server, trying to build an enterprise-wide solution to capture quarterly financial based budgets and/or forecasts from every budget-holding manager across the organisation, for consolidation and approval by the CFO. The issues you would face would revolve around data-capture, workflow, security, consolidation, business rules, performance, tracking etc. Even if you were successful, it's likely that you would eventually end up in Excel hell, or left with no confidence in the results and a huge team on full-time support.
Step forward PerformancePoint Server Planning. It removes much of the pain associated with all of the above and is not just limited to financial based scenarios. In addition, when integrated with PerformancePoint Monitoring you can track the actuals against the captured plans/targets from Planning to ensure the business stays on track. When the enevitable deviations occur, the Analytics element of PerformancePoint Server (Currently ProClarity) will help analyse why to allow corrective action to be taken.
For a little more detail..