WHY does INS want to do interview at local office during I485 processing?

[gc2k]

Could someone please share your experience and knowledge on my questions:
1. WHY does INS want to do interview at local office during I485 processing? What do they most likely ask? (EA case)
2. Does INS just want to check I am always in legal status?
3. Does recent anti-terrorism make INS to ask for more interviews?
4. Does INS want to check whether I work in the field that I petitioned in I140 EA case? (Doesn\'t it belong to I140 process?.)

Thanks!