is it correct or necessary to uise a comma before the word "and" prior to the last phase in a sentence?
Do you mean:

"I ate apples, bananas, and grapes."

The last comma is sometimes called an Oxford comma.

I love it. It makes such lists clearer. Use it.

Doesn't the sentence "I ate apples, bananas and grapes." imply that bananas and grapes are somehow related? It does to me. The Oxford comma lets you make sentences like: "I ate apples, bananas and whipped cream, and toast." clearly implying that the whipped cream and bananas are served together.