You can picture it as a sort of conical volume. When you focus on the ground plane, the top of the flowers which are next to the camera might be out of focus, while the top of the distant mountain will be in focus. Esthetic considerations determine whether you use front tilt, back tilt, or a combination of both. However, if you use long lenses, you might find that front movements require monkey-sized arms to operate. Front tilt preserve linear structure, in particular keeps the verticals parallel.
Front tilt requires more lens coverage than back tilt.
If your camera has both, its probably better to stick with one of the methods. It is a matter of personal preference, and doesn't affect the final image. With base tilt, it seems clearer to me which areas are out of focus. I also find axis tilt confusing if the far point is not close to the axis of the tilt. With base tilt and shorter lenses, you might find that you cannot get the two standards close together enough.