The idea is to more evenly distribute people stowing bags in overheads up and down the aisle. If there are ~160 seats in economy boarded in 4 zones, better to have each batch of 40 spread out over all ~30 rows rather than clumped together fighting over 8. Put another way, the you'll be stashing your things in the compartment over your seat with 1 or 2 other people, not 5, which should mean less waiting and thus less of a problem with blocked aisles. The elites all board at once of course but in theory, they should be familiar with stowing things in overheads, and perhaps have developed a system.

In practice? Being in Zone 1 I can't tell the difference . Sometimes I think it would help having more zones, except they already call multiple zones sometimes, so it already seems like the entire universe is boarding (especially on US Airways, which gives priority boarding to holders of their credit card!).