Advance the timing before TDC. Why?

Hello guys. My question is to learn the purpose of what I did.
This is the engine; 318ci, bored .020 over, stock pistons, 302 heads decked to 58cc (ported), Edelbrock Performer intake (ported), Edelbrock 1406 600cfm Carb, Comp cam XE262H Duration 262/270, Lift .462/.470, stock distributor

It was recommended I advance the timing on the engine when I tune it. Stock tune is 0 degrees, or TDC. I've tuned it to about 9 degrees before TDC.

Why do I want it tuned that way?
Still related to ignition. What would you suggest for a distributor to replace stock?