As an altar in our sense, as a place for burnt-offering, the obelisk could scarcely serve, but, when it stood on a grave-mound or on a basis, mound or basis would serve as altar while wreaths and stemmata as on the coins would be hung on the obelisk.
Harrison argues that the obelisk functioned not as a sacrificial altar in itself but as a vertical marker atop the tomb-mound, whose base performed the altar-role, collapsing the distinction between commemorative stele and sacred pillar.
, Themis: A Study of the Social Origins of Greek Religion, 1912thesis