Arc junctions are only at nodes. Vectors just seemed more correcter. TIN must be used to represent volumes. Vector can represent point, line, and area features very accurately. Vectors are far more efficient than grids.
Vectors work well with pen and light-plotting devices and tablet digitizers.
Vectors are not good at continuous coverages or plotters that fill areas. Topology allows automated error detection and elimination. Rarely are maps topologically clean when digitized or imported. A GIS has to be able to build topology from unconnected arcs. Nodes that are close together are snapped.