I'm with the mob who say it's just tasty!
There is also apparently evidence to suggest that it is not likely to be diet related - a nutritionist was telling me of studies, but can't remember the details.
But... Firstly, if she's not getting enough roughage, then it could well be because she's just hungry. Even dieting horses need around 2% of their bodyweight daily in forage & it's not good for them to be left hungry.
If you're worried about obesity/IR (which is a very common problem), you can soak the hay first, to leach off some of the sugars, &/or you can provide hay free choice but in a 'slow feeder' small holed hay net. Grazing muzzles are also another great option, for reducing grass intake without making them go hungry. Safergrass.org & ecirhorse.com are 2 good sites to learn more.
Re nutrients in general, yes, if she's just on grass & Mg, then IME chances are she will be deficient/imbalanced in at least a few things. But every feed mob wants you to think their feeds & supps are the necessary one, some are far better than others, and it depends on what your horse gets now as to what she may need(or not) supplemented. So I do believe it's important to do a basic diet analysis at least, before you buy. FeedXL Horse Nutrition: The D.I.Y. Equine diet planner
is a great Australian service/resource for helping with diet & nutrition.
Re Ca:Mg balance, while I do know that Ca is one of the 'biggies' that has been considered necessary, there has recently been a lot of research done on Mg & it's been suggested that the accepted Ca:Mg is far from ideal & that a lot more Mg is beneficial. The more I learn about it & the more I've experienced horses supped with Mg while Ca is kept lowish, the more I'm inclined to agree.
So... as you can see, I'm not qualified or well read enough to be sure of the answer, but IME it's well worth looking into more & considering that a Ca free or low supplement may be the way to go, along with providing more Mg. Gravelproofhoof.org is one place to learn more magnesium website