A small angle can make the whole frame feel calmer.
A photograph can look unsettled even when the camera was only a degree or two off. Horizons make the error obvious, but door frames, shelves, and windows can reveal it just as quickly. Fine rotation is for that small correction, not for turning every line perfectly vertical.
Use the 90° buttons when the photo orientation is wrong, then use the angle slider for the last adjustment. FilmFX automatically zooms just enough to cover the corners created by rotation, and the final export is rendered from the original source rather than a CSS-transformed preview.