explaination given to me was what if I485 gets approved when u r abroad...then technically you are not adjusting status
read this ....posted earlier!!
"Given the unpredictability of processing times, it is possible for the I-485 to be approved while one is traveling abroad. Therefore, although H-1/H-4 and L-1/L-2 holders can travel without advance parole, it is best not to do so. This is particularly true for individuals who do not have valid visas in their passports. If the I-485 is approved while an individual is abroad, s/he is no longer eligible for nonimmigrant (temporary) status. The best situation is for the individual to have advance parole, which makes travel possible, and to explain the fact of approval at the Port of Entry. The worst situation is if an individual does not have advance parole and plans to obtain an H or L visa at the consulate for reentry. If the I-485 is approved while this person is abroad, s/he is not eligible for the H or L visa at the consulate. Without other documentation permitting travel, one would not be allowed to board the plane without a travel document or valid visa stamp in the passport."