As people have already said many spells like fireballs, comet falls and so on require a reflex save to halve their damage.
When these spells start hitting for a third of your HP and can glue you to the ground (comet fall) having an acceptable save is a good idea.
However as there is no point going THF on a Barb you should be starting with a 15 dex anyway unless you have a +3 dex tome banked.
To the OP - max both Power rage lines