Zero-lot-line homes do not share a wall. Each building is constructed such that a wall can abut the property line. Duplexes do share a wall. The two types of building are distinct.

Rowhouses/townhomes would be homes with two opposed zero-lot-line walls, exactly the width of the property, all in a row. If a whole-block-width building had partitioning walls at roughly the same interval, that could instead be a rowhouse-like condominium.

Sharing a wall does bring in some legal issues and definitions, which do not come into play when each structure is completely independent (but abutting other structures).

The ability to build without setbacks is a major factor in urban density. The costs of maintaining a city roughly scale with the linear amount of street frontage, so not wasting that length on empty setbacks is a great way to keep city services cheaper.