We divide by 1000 to turn that term into kilo pascals (kPa). Without that the first term would be in pascals. I could, of course, have simply multiplied the second term by 1000 to get it into pascals before adding the rho*g*h term, but would then have had to divide the result of the summation to get it back to kPa, as the possible answers are given in kPa.