Yes you are not removing you are adding .. so you have to build on top of something and if your part has thing with nothing under them then the slicer has to build some support under that area.. You have to consider orientation when printing as to which makes the most sense and which will produce the strongest part. Here is an artificial from the simplified 3d website that explains supports.. even if you are not using S3d there are many articles that may be helpful to you on their website.. BTW the current version of their software has been very broken since they updated it past 4.01.. Hope that helps