After you create a segment, you may want to iterate on it down the line without changing the conditions. This is when you might want to build a similar segment, which is essentially a clone of your original segment. You can then add or change conditions depending on what you would like to do with this new segment.
For example, if you have a segment of recent purchasers and you'd like to create a related segment of recent purchasers who also made at least two purchases in that period, using the Create Similar Segment option will save you from having to setup the same conditions for recent purchasers again.
How to Create a Similar Segment
You can find the similar segment option by selecting an existing segment and clicking Manage Segment > Create Similar Segment.
Then, you will be brought to your duplicate segment's definition menu, where you can add or delete conditions. Make sure you give your duplicate segment a new name to avoid confusing it with the original.