Data in the 1-15 GB range is more common than you'd think. Spark is overkill and comparatively a nightmare to configure. You can fit 15 GB in memory nowadays on a single core, but you probably want things to move a little quicker.
But even in a small cluster type of use case, I think the main users are scientists or other practitioners who, for whatever reason, don't have the support of a platform team or data engineering team that can keep a Spark (or other) cluster fed and cared for.