I had a look in the manual of QGIS and I also browsed the Internet to find some useful information about the geometric functions of the field calculator.
I've found How to use the Intersects function in the Field Calculator of QGIS? but I'm still confused.
Does anyone have some practical examples to add?
I think all these functions could be very useful, but at the moment, their usability is quite difficult for a 'normal' user.