You lower the DCR because you need to lower the DCR. Lowering it in an existing/working crossover can be detrimental.
Working with DCR can help you compensate for baffle step effects without a separate baffle step compensation circuit.
One major reason you might want to go with iron core is that otherwise your coil would be too big. If you need high inductance, low DCR and smaller size/cost iron core is the way to go. More inductance = more windings, longer wire. Naturally, more wiring would normally add DCR so at a certain size of indcutance, like you might need with a very low frequency low pass filter a standard air core may be too big or heavy or expensive or impossible.