Most work can be clamped directly to the table using the T-slots.  The
removable fences allow the work to be aligned to the edges of the table
which, in turn, are aligned to the x and y axes of the mill when the
block is clamped in the vise.  Making the fences removable (after the
workpiece is clamped in place) ensures that they won't be damaged during
the machining operations.