Is there a reason to get chalk powder instead of chalk blocks?
I think its just a matter of preference. I personally order very cheap chalk blocks online and just break them up in my chalk bag.
Some climbers prefer chalk powder because it is more convenient to use, allows for better distribution on the hands, and creates less dust. Others may prefer chalk blocks because they like the texture or find them easier to handle. Ultimately, both chalk powder and chalk blocks serve the same purpose of improving grip by absorbing moisture from the hands.