I wonder why Newark has so many? You'd think Logan or certainly Dulles would have more international arrivals.
JFK, LaGuardia, and Newark also all have hourly limits on the number of flights at the three airports. LaGuardia has the perimeter rule, limiting international flights to destinations in Canada and the Caribbean. JFK is limited to 81 flights per hour and many of those flights are to domestic destinations, so Newark serves to accommodate some of this international demand that can't be met at JFK.
Newark also has a different catchment area than JFK. For international flights to Asia, Europe, and other destinations, I wouldn't be surprised if Newark can attract some passengers from the Philly-area.
Finally, as noted above, Newark is a huge hub for United/Contental, so there are many connecting passengers from other places in the US. Logan on the other hand, is almost entirely origin/destination traffic.