The 747-400ER freighter can fly 525 nm farther OR carry 22,000 lbs more freight than the standard -400F. Either can mean more income per flight, depending on whether you want to carry more freight, or make the planned flight distance without a fuel stop (which are costly). BTW, the 744F-ER can now carry 272,000 lbs. of freight, whoa!
I read recently that 15 744-ERs are on order, and about half of them are freighters.