[shliachtx]

Another important point is that the eruv was instituted because of the rabbinic prohibition on carrying in a semi-public area.

According to Jewish law there are 4 categories of domains: 1) Public (Large area, heavy traffic), 2) Semi-public (most of what we would consider public), 3) Private, 4) Permitted (a small distinct area in the public domain). Carrying in the semi-public domain is biblically permitted, but rabbinically prohibited due to its similarity to a public domain. An allowance was made to carry in the semi-public domain if a symbolic eruv is put up to distinguish it from a public domain.