Specifically less dust if you have filters on the intakes. Positive pressure means you'll have air coming in where the fans are blowing it in (through a filter), and any gaps in the case where there aren't filters will have air flowing out due to the pressure.

If you have negative pressure you'll be sucking in air through the gaps and that air won't go through a filter, hence more dust.

Is this really part of the ATX spec though? Or just something people have learned to do for modern cases with air filters?