Advance Keys

OK, so I have another 360 I'm toying with. I have never used the offset keys, but am wondering if/what the advantages are.

I have a Cloyes timing set with no provision to advance or retard timing.

SO, what are the performance advantages.

This will be a stock bore 360 (about 9:1 or a tad better compression), Speed Pro pistons, Comp XE268 cam, LD340 intake, factory J heads with a mild porting just for clean up,--carb undecided--.......