Well the title says it all...
The ideas which are modified, due to something that you were not aware at the time of posting idea. So you add this info afterwards, should be reintroduced on latest ideas list.
For example: I posted this idea: http://brainstorm.ubuntu.com/idea/8251/, but forgot to write how to actually do this idea.
So I took time to think about it.
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this is good, because it allows you to insert new info about your idea, which you got from the comments or on net, even if the idea was posted long time ago.
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Some of this info can completely change the whole point of idea or improve it or make it easier to understand.

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This idea could be a bit confusing, but sorry I can't put it better in words than this.