Glycine’s not a filler in this instance. It’s used to aid absorption [1]. Mag glycinate is one of the only worthwhile forms of magnesium for supplementation, unless you’re using it as laxative. The latter being caused by gut irritation from unabsorbed magnesium.

But yes, glycine also has many of its own benefits. Including a very interesting theory regarding life extension due to its interaction with methionine. [2]

[1] https://examine.com/supplements/glycine/#nutrient-nutrient-i...

[2] https://www.fasebj.org/doi/abs/10.1096/fasebj.25.1_supplemen...