Glucoamylase is more effective at saccharification than the beta amylase in malt.
It’s common for the amylases in malt to create nonfermentable dextrins. Glucoamylase can reduce these dextrins to fermentable sugars - thus giving you a higher yield when combined.
its also common to use high temp fungal amylases during cereal mashing, where regular malt amylases would be quickly denatured by the high temps.
Use them alone with unmalted grains, or together with malt, there are good reasons in both cases.