By default, Project calculates variance comparing current data against Baseline data -- not Baseline1, or Baseline2. So, the ability to copy a baseline into one of the "spare" baselines and then re-baseline is designed to take advantage of the built-in calculation.
The reasons why a project manager re-baselines are many. In most circumstances it is negotiated with the sponsoring organization as it is used for comparison.
If you re-baseline after tracking any variance created by tracking is removed when the new baseline is saved as it sets Baseline data = actual data on the tracked tasks.
I hope this helps.
Julie
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