The sense of 'hieratic' you encountered in the Wikipedia article is this:
c. Applied to a style of art (esp. Egyptian or Greek), in which earlier types or methods, fixed by religious tradition, are conventionally adhered to. Also fig.
["hieratic, adj.". OED Online. March 2016. Oxford University Press. http://www.oed.com/view/Entry/86793?redirectedFrom=hieratic (accessed March 12, 2016).}
Although the application of the term to a style of ancient Eygptian writing predated the general and figurative sense given above (1669 as compared to 1841 for the earliest attestations of the respective senses in OED Online), the word itself derives from Greek "ἱερᾱτικός priestly, sacerdotal, devoted to sacred purposes" (op. cit.) via Latin hierāticus.
I am scarcely qualified to interpret religious iconography with any degree of sensitivity, but the "hieratic purposes" of the footstool may involve
- the convention, fixed by religious tradition, of depicting enthroned Mary with her feet on a footstool in prior religious art;
- the multiplex religious symbolism of the footstool.
The religious symbolism of the footstool arises from biblical verses to the effect that the earth is God's footstool, and that strong belief will permit you to rise above, that is, conquer, your enemies:
- "Thus says the LORD, "Heaven is My throne and the earth is My footstool...", Isiah 66:1; "...or by the earth, for it is the footstool of His feet...", Matthew 5:35; etc.
- "...Until I make your enemies a footstool for your feet", Luke 20:43 and Acts 2:35; "Until his enemies be made a footstool for his feet", Hebrews 10:13; etc.
Thus, the "hieratic purposes" of the footstool are to evoke in the faithful, by conventional use of the footstool symbol, the biblical connotations of the footstool as outlined by the quotes given above.