What is it with all this re-branding?

First of all "re-branding" is more often termed brand extension or product modification. There are differences between the two.

When a product is modified, it's often because of new features. To promote these new features or benefits, the products' name and packaging is changed. You mentioned Marathon vs Snickers... Marathon is an extension of the original candy --marketed as an energy bar.

Also because of these modifications (of Marathon as an example), Snickers hopes to gain a larger share of the candy bar market. By adding Marathon, Snickers is trying to capture the health sector (or consumers that are health conscience) of the market --otherwise known as extending the brand.

Thus I disagree with ad agencies being responsible for modification and extension of a company's products. They have responsibilities to maintain or shape a brand's image. However their work is not without great review from the client's marketing/managing department (especially big-name companies like Snickers). Product re-modification is often the clients' responsibility. The ad agency more or less makes decisions regarding the brand, in respect to their clients' product initiatives.

It has everything to do with advertising agencies wanting to make more money. After all, if a company like Doublemint Gum doesn't change its jingle or wrapper in 100 years, that doesn't leave much opportunity for an advertising agency to make some money.