As allways it depends from how and what you want to model!
Normally you don't need masking.
You need it when:
- you don't want to model something in your images.
- Photoscan has trouble with parts of your images (e.g. reflections, ....).
- changing background (e.g. moving objects, ...)
- ...
When you use a turntable the background is static but your object is moving, so if PhotoScan is using the background for image alignment it can't align your images properly, all images seems to be shot from one position (as they are in reality) instead of a circle around your object. Here you can use for example masking from (empty) background image to generate masks for all images but others have also good success with an uniform (overexposed or black) background, where PhotoScan can't find tiepoints, without masking .
For a turntable tutorial with masking see:
https://www.youtube.com/watch?v=9_F-b2hxP_oKarsten