Sports drinks have been making various claims of superiority to plain water for a long time, and I've been generally skeptical of the whole lot of them. Still, if you're chasing someone who's got a pocketfull of Gu gels (and you don't have any), you do tend to wonder how much of a benefit there is to the various energy-boosting and/or electrolyte-replacing products available today.
When hiking high-level endurance objectives, and especially so in hot weather, I'm 100% onboard with the idea that you've got to carefully manage your salt intake. In fact, I salt my salty snacks nowadays, because I've found it's often difficult to get enough salt without resorting to extraordinary measures. Probably just with plain water and the right mix of salty foods, you can duplicate much or even all the benefits of Gu-type products—but it's probably easier to go with Gus.
I find palatability becomes a real problem as marathon days wear on in crushing temperatures. Your body begins to recoil against the idea of eating and even drinking. The mere thought of eating solid food can become nauseating, and plain water loses its appeal as well. Worse, eating super-salty snacks or even pure salt can quickly upset your stomach if you overdo it.
In these situations, adding some drink mix to your water can really help, because it makes fluids appealing again. And if you pick something like Gu's double-salt blueberry mix, you get a tasty drink that is also adding significant sodium without goofing up your stomach. So that's become one of my go-to products for exactly these sorts of situations.
As for the gels, they do seem to provide at least a psychological boost, and maybe they help keep cramping at bay also. I haven't seen evidence I'd trust to definitely claim they work as advertised, but in my own anecdotal experience they probably don't hurt, and they very likely do help give you a lift, mental or otherwise, when things start getting desperate.