A lot of ads should show us that we need to be very careful about the negative side of the society we live in. As a capitalist society, when it comes to business, people are basically in it for themselves and have very little incentive to be truthful just for the sake of being good. When we look at ads critically, we see that this is how our system works, for better or for worse. The more knowledgeable we are about that, the better off we'll be.
Also, don't discount the humor factor. Many ads give us something to laugh at, which is not always the advertiser's intention.