These foot archers are raised from settled former nomads and will give useful service on the battlefield thanks to their good bows.
Unlike most other Sarmatians, many of the bowmen from Sarmatian groups that have become settled do not shoot their arrows from horseback, but fight on foot. Certainly, foot archers lack the mobility of their mounted counterparts. In exchange, men on foot need less space than those on horseback. Thus, foot archers can be deployed in tighter formations that provide heavier arrow barrages. And the composite bows that these foot archers employ maintain still the qualities of those of their nomadic relatives.
Historically, several groups of Sarmatians became sedentary when they moved into areas (Ciscaucasia, the Hungarian Plain, etc.) where farming was a viable option to their traditional pastoral nomadism. The Siracae, for example, were a relatively small Sarmatian group that occupied the southern edges of the steppe along the Black Sea and in Ciscaucasia. Since quite early, they initiated a process of settlement and hellenization and, while still fielding armies with numerous cavalry, they resorted to infantry most than any other Sarmatians in the centuries BC. Presumably, the abundance of horsemen may have not been the only trace that steppe warfare left upon the ways of waging war of settled Sarmatians and substantial numbers of their infantry seem to have been archers using composite bows.