Doing most of the FA work in parallel to admissions work, rather than waiting until after admissions is done, means that there is less of a time crunch.
The likely process is that FA applications are fed into a computer program that makes preliminary FA packages based on college FA policy and flags unusual situations for human review. These unusual situations may take some time to figure out, so doing that in parallel to admissions work means that if admissions is done on 3/31, the decisions with FA packages are ready to be delivered on 4/1.
Of course, the cost of CSS Profile is a separate issue / complaint (but the above time framing also applies to colleges that do not use CSS Profile).