The gastraphetēs (hand-held crossbow) and oxybelēs (oversized version requiring a stable ground and a crew to operate) were probably invented by the engineers assembled by Dionysius I of Syracuse in c. 400 BC. From Sicily these weapons spread to Greece.
Their usage remained mostly restricted to siege warfare, for several reasons. The expertise needed to produce and maintain them meant only those city-states with proper arsenal had them. In pitch battles their cumbersome shape and size, i.e. poor mobility, and high reload time meant they're inferior to light troops (javelineers, archers, slingers). In siege warfare, however, moving around and rapid firing are unimportant, due to the fixed place and long duration; there they had a clear advantage over traditional ranged weapons: similar to firearms, using a crossbow meant you had to expose much less of your body for a short period, since you could reload behind parapets etc., in contrast to slings, bows-and-arrows, and javelins, which need more space for human arms to move around.
Alexander the Great is said to have deployed gastraphetēs during the Siege of Tyre (332 BC). A mace crossbowman unit has been created and added to the public mod some time ago, but is not used yet. This patch enables training it at the siege workshop.
Their current stats are very poor, but are not changed here; see D2886 for that.

Agree.