I actually use both of these features when processing my mesh files prior to uploading them. Reducing the triangle count is called decimation (though not sure what it has to do with ten), and it should only be needed on certain textured parts. E.g., textured parts from SolidWorks. These have a constant mesh size regardless of surface curvature. Decimation will reduce triangle count on flat and large radius surfaces. The advantage is that the upload files are a fraction of the size, no other gains really.
The slicer is pretty robust, and usually holes in the model will not create an issue despite the warnings. If you have issues, than the repair tool can help (but usually when things are that bad, the tool can't fix it).
In summery, if you are having no issues slicing the part, then no need to use these tools.
