[barrkel]

Writing code that writes code, where that written code is executable at machine speeds rather than interpreted, is a qualitative difference over a simple "eval" implementation; and it can most definitely be a feature of the language.

A language which supports it as a feature may do things like partial evaluation of closures, inlining of runtime-constructed code, transforming e.g. the typical block-passing idiom of a Ruby into specific optimized cases. Being able to rely on such transformations in turn lets you build more abstract (i.e. more highly parameterized) libraries because you know you won't have to pay the same degree of abstraction tax.