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