Why advance timing?

I posted something about the longtime overheating issues with my stock 225 automatic (20k miles on the rebuild) in another thread, but I think I just resolved things by advancing my timing to 5* and idle to 800.

I'd previously had it at TDC, idle somewhere between 700 and 750 (750 is spec according to the sticker). Dwell is at 39, which is slightly below the 41 to 46 spec, but it's always a crap shoot for me to get it close, so 39 makes me happy. :P

I'd tried a new radiator and cap, new water pump, new thermostat, and only gotten incremental improvements.

So why do I care if I've "fixed" my problem? I want to make sure I'm not just masking an underlying issue by advancing the timing. I've seen a bunch of posts about advancing timing until it pings. Shouldn't a stock motor be able to run at the original spec of TDC timing?

Thanks, all.