With a heavy arrow like hickory or ash do you need less spine than with a cedar shaft? I am new to the wood arrows and it seems like it would be like adding weight to the back of a carbon to stiffen it up.
I made a couple arrows from dowels and they spined within 2# of each other but one weighted about 50grs more and it took more point weight to get it to fly true. Did the extra arrow weight cause this??

I'm not sure why you'd need more point weight on the heavier one, except for FOC issues.  Heavy wood arrows do seem to fly as if they have a lighter spine, but it's not much different in my experience. Maybe only 5# or so.  I've not shot hickory, just ash and birch.

Generally when you move from POC to ash arrows, you need to go UP in spine.  The short answer as to why is the heavier arrow makes your bow slightly more efficient, so more energy is transferred to the arrow.