I don't think this has anything to do with manually evolving a baseline. I'm guessing that overides any manual setting. I think this is to stop automatic evolution of that baseline. SO let's say you turn on automatic baselines, but want to make this plan an exception from that automatic evolution.
Just to clear up a point here, I believe the key to this deature is not to let things progress automatically. If you want things to change over time, you might as well not use baselines and just let the optimizer do it's thing. I feel the true value in this feature is to prevent degeneration. I mostly use SPM during upgrades. When we see a reduction in performance from a move between 10g and 11g, it raises alarm bells. What do I do? I set the optimizer_features_enable back to 10g, run for a while, capturing the baselines. Then switch to optimizer_features_enable of 11g and keep using the original baselines. Then manually evolve anything that looks like it might be improved in 11g.
I spoke to Maria Colgan (from the optimizer team) about this and she agreed it is a common usage.
In my opinion, and I could be wrong, letting this feature run in automatic mode is a mistake. This is something to stabalize the situation and allowing automatic capture and evolution does not stabalize the situation in my mind.
I've been to a number of talks about this feature over the years and few of those talk about this feature in any sense other than what I've just described. Once again, it is just an opinion.