It really depends on how the combos branch out. If a single button takes you through all the steps of a combo, the instant that you press that button, you've committed to your positional two GCDs down the line. Having individual buttons allow you to defer that decision until just before your positional step, which in turn allows you to potentially escape into another combo if that positional is temporarily not available. It just offers a bit of extra flexibility for PvE gameplay.