The broad general training of GPT-X (and fine tuning on your content) provides context and (loosely speaking, at least) “analytical” ability, search-via-embeddings to inject material into the prompt provide exact recall of specific material, with capacity greater than the context limit.

Analogous, more or less, to a human with general experience (base training), experience with your code base (fine tuning), and the ability to reference the current code base directly (embedding-based search/recall). All three have a role, they are complementary rather than mutually exclusive.