The reason for the tour is probably due to the fact that we only have one tour coming to NZ this summer.
Normally we'd have two tours, one kind of big one (5 ODIs, 2 T20s, 2/3 tests), and one smaller one (3 ODIs, 1 T20, 1/2 tests).
NZC will get most of their revenue from T20s, followed by ODIs.
By having 3 T20s that means they still have the amount they'll normally have with two tours, hence not miss out on the money.
With the six ODIs it's a mixture of them heading into the world cup, and because the want the extra revenue. Normally we'd maybe play 8 ODIs in our summer, but by playing 6 they're not going to lose out on much.
Test cricket in NZ makes very little money (less people and cheaper tickets), so by having less of them it won't effect NZCs revenue much, and heading into the world cup they probably decided to have a 6th ODI ahead of a 3rd test.
That's just how I see it anyway, the 3 T20s will be more or less purely for revenue purposes.