It reduces disk space.

Historically the dyld_shared_cache has been a pre-bound copy of most system libraries that is mapped into memory once and then shared across all processes. It was updated automatically by the system at various times, and if a given system library was determined to be newer then the dynamic linker would simply load it from disk. The result of this was that for most system libraries you ended up with two copies on disk: one at the normal location, and one within the shared cache. Now only the latter copy exists, and Apple ships it directly rather than creating it on each machine from the copy of the system libraries that they ship.