Legally, the advertiser needs the release in hand to use the model's likeness for commercial purposes. The photographer doesn't need the release, as they're not the one using the image commercially.
However, since the photographer is often the one who's directly interfacing with the model, it's usually just simpler for everyone for the photographer to get the release from the model. In this case, the photographer should make sure the release allows the advertiser (not the photographer) to use the image, OR the release is worded in such a way that it's transferable from the photographer to the advertiser.
If the model already has a relationship with the advertiser, then the advertiser can get the release directly from the model. But this isn't always the case - often, the photographer is the middleman, and the advertiser doesn't have any direct relationship with the model.
Source: Internet Armchair Lawyer, Esquire