Why circumcision?

ASKING THE IMPORTANT QUESTIONS: Why Do Jews Circumcise Their Sons? In First Temple times, almost all the region's peoples were circumcised, which indicates that the roots of the practice lie deep in prehistory (Elon Gilad, Haaretz). The article concludes:

Looking further into the past

So it seems that Jews circumcise their sons because their ancient Semitic forebears did, but why did they start circumcising in the first place? These ancient Semitic people didn’t write, so we can’t know what they were thinking, but we can speculate.
Since circumcision was carried out on the sexual organ, and probably at puberty, we can assume they thought it would improve fecundity.

Indeed, fertility is exactly what is promised Abraham by God in return for circumcision.

But where would these ancient Semitic people get the idea that cutting their foreskin off would improve fertility?
The answer may be from their farming habits. Archaeological evidence shows that the farming of grapevines and olive trees was spreading through the region during this period. These plants require regular pruning to increase yields. Maybe some ancient Semitic sage came up with the idea that if pruning vines increases yields, why not prune penises too?

In fact, there is evidence in the Bible that the ancient Hebrews tied circumcision to pruning “And when ye shall come into the land, and shall have planted all manner of trees for food, then ye shall count the fruit thereof as uncircumcised [literally: ye shall foreskin their foreskins]: three years shall it be as uncircumcised [literally: foreskins] unto you: it shall not be eaten of” (Leviticus 19:23).

If this is all true, Jews circumcise their sons because an ancient tribe converted an agricultural innovation into a questionable method to increase male fertility, and later a small group of their descendants bestowed this practice with a national meaning, which endures to this day.

Interesting article that is rife with speculation, but within a plausible range. One detail: in linguistic terms, Arabic is a South Semitic language, different from both East Semitic (Akkadian) and West Semitic (Hebrew, Aramaic, and related). Ethiopic is also South Semitic, as is Old South Arabian. The isogloss that separates out South Semitic is the "broken plural" (or "pattern replacement") of the noun.