Is advertising really that important to a presidential campaign?

I don't understand why each candidate would spend the equivalent of a small countries entire economy just on getting votes...
It seems like such a waste when there are bigger issues money of that capacity could be spent....

Is the margin of undecided voters that large? Or does their budget also include encouraging people to vote as well... it's not compulsory to vote in the US is it...

It's just mind boggling the $$ injected in just to see one person elected....


"He's my best friend, best of all best friends
Do you have a best friend too
It tickles in my tummy
He's so Yummy Yummy
Hey you should get a best friend too" - Toy Box