The road beyond Domitian's gate was flanked by an enormous number of funerary tombs of different shapes and sizes.
Archaeologists have come to the conclusion that some of these monuments were built as part of a propitiatory (atonement) rite:
sick (and wealthy) people who came to Hierapolis to recover their health,
paid to have a tomb in the hope that they would not eventually need it.
That may also explain why some monuments were decorated with phalli, a symbol of life, rather than death.