This practice can vary between countries.
I fly a lot out of Greenland and there you can file VFR flight plans over the radio on the ground or just after take-off, so called GLP. There you are expected to give POB including pilot(s).
However I also fly in Iceland and there you are expected to inform ATC the name of the commander + pob (not including commander).
Can get confusing I know, but many countries / many rules
The purpose of this is in case of emergency that the search and rescue folks will know how many bodies to look for or if a rescue heli is sent, that it will be able to carry everybody.
edited to include answer to question #2!