Because your product is exactly the same as everybody else's. Faced with a choice between Axe, whose ads give off an image I don't like, and Old Spice, whose ads give off an image I do like, I might pick Old Spice.

I'm on a horse.

Seriously though, do you actually not understand that not everybody in the universe is hyper-rational and will occasionally make purchasing decisions based on ineffable considerations of "image", or are you merely rhetorically whining that not everybody in the universe is as hyper-rational as you imagine yourself to be?

I find that telling jokes is a better way to pick up girls than writing a proof as to why I am a superior mating candidate.

Weird, huh?

If deodorant were a new product that solved a new problem, the informational approach would be appropriate. But since deodorant is a commodity and they all pretty much work, this kind of marketing isn't effective anymore, so the brand-building marketing begins.

Because their brand isn't better than the competition - it's just different. Old Spice and their competitors all have pretty much the same ingredients (save their scents), and work pretty much the same - and if one is slightly better, it's so little that you wouldn't lead an ad with that even if you could find a number.

Since they can't sell a concrete product as better than another one, they're selling the associated image - and betting that you will buy one of many indistinguishable products (theirs) because you like that image.

It's like Coke vs, Pepsi (or Sprite vs. 7-Up) - in a blind test, they taste pretty much the same. One isn't empirically better than the other. So instead they sell the images the drinks associate and the brands, and compete in that sphere.

Because most people don't care about scientific studies, and your competitor can also "fund" and "report" on a scientific study that favors his product.

"Why advertise this way?"

How else is your brand going to be talked about incessantly and shared over the Internet by millions across the world? All in a 24/48 hour period. No other brand is currently being talked and raved about currently and in the same volume.

I'd love to see the effect this campaign had on their sales.

Other crazy factors involved:

1. You can't smell the product when you're in the store, so you have to know more about the product either through brand awareness or referral.

2. Like cologne, the smell you give off from a deodorant is about projection. Who you want to smell like is probably going to be more of a priority to potential customers than what you want to smell like.

This would be just like shaving blades which were mentioned on here the other day, they tend to compete on supposed advanced function when it just becomes a stupid game of adding one extra feature all the time when we just want something that works for shaving.

I've seen ads here recently for deodorants that change smell over time, I guess that's one of that gimmiky "features".

Also these items are in the price range where I don't really want to waste my time thinking much about pros/cons so this advertising becomes more effective. As opposed to an item like a computer where I would analyse the specs of most of the components making advertising less useful.

Business school exists for people like you.

This seems like a perfectly reasonable question that might stimulate interesting "meta" discussion about marketing.

There is an old advertising maxim: "Sell the sizzle, not the steak."

The maxim exists because selling the sizzle works much better. The current Old Spice ad campaign is the best modern example of selling the sizzle.