as Belgarath says... "raising those little boys was not the only thing she was born to do, but it was one of the most important".
Polgara can use the Will and the Word because necessity needed her to. I think in many instances she doesn't use her Will as such - rather her eons old wisdom and her basic, innate mental attributes. She's extremely intelligent, she's extremely beautiful, and she's been alive for thousands of years...