One of the important things about employing full stack engineers at small companies is that the skillsets of frontend and backend don't always meet the needs of the work they're doing.
Consider two examples: sending an email, which requires typical frontend skills but is by nature in the backend code; and configuring invisible backoff and retries in the frontend, which is the opposite.
I call myself a full stack engineer even though my ability to do design-adjacent work is pretty bad because I've spent a lot of time working in this middle ground. When you want to e.g. move PDF generation from the server to the client, or you want to untangle the horrible Redux caching layer that doesn't invalidate things properly, that's a specific blend of skills that's neither fully one nor fully the other. This is less about cost and more about a niche between the two fields.
Consider also data engineers, or teams with a technical lead rather than an EM, or any other role that sits between worlds.