A baseline is a snapshot of the plan at a fixed moment in time. It is the original approved version of what you set out to deliver.
You do need to manage the project using the most recent plan. But it is useful to have a copy of the original baseline for comparison. Here are some reasons why:
A baseline plan can show you progress. You can compare the actual progress made (as recorded in your up to date plan) against the forecast progress from your baseline plan. This will give you an idea about whether the project is on track or not. You can compare task slippage, changes to milestones and see how much the end date has moved. These are all good things to include on your project status reports.
If you have kept a copy of your baselined, original plan, you can compare the original estimates to the time it actually takes to do the work. You can record the differences and let your Project Management Office know, along with any other project teams who are currently estimating for their projects. When you come to estimate similar tasks on future projects you will have a much better idea of how long things will take in reality, based on your experience of this project. You’ll produce better estimates with your team because of what you learned from this project and your comparison back to the baseline.
A baseline plan is essential for a successful post-project review. It is a really useful piece of documentation to have available at the point where you are assessing how the project went. You can see all the changes and how they impacted the task durations and the resources involved. You can see how the estimates evolved over time, and what the impact of your estimating techniques was.
You can take more than one baseline snapshot of your project schedule during your project. It is definitely helpful to have one at the beginning, but if your project includes different stages or phases you could take another baseline at the start of every stage