A dead witch can appear to a living descendant in a dream, or as a hallucination or blur, as well as possess them.
A powerful dead witch can allow supernatural beings on the Other Side to gain a foothold on the world of the living, giving them the power to interact with the physical plane of the living. In turn, a living witch can perform a spell that will reveal them to everyone on this plane, not just mediums .
A powerful dead witch can resurrect themselves when the veil is down.
If her body is preserved, a witch can return to it from The Other Side. This requires channeling a great amount of magical power.
In Ghost World, Bonnie casts a manifestation spell in order to determine what was wrong with the other side, allowing ghosts to interact with the world of the living. It was said to reveal ghosts, also known as Veil Matter.
In Graduation, Bonnie, now dead, raised the Veil back up channeling the Full Moon. Once the Veil was back up, all of the Ghosts lost their corporal state. Sadness struck as everyone is gone now.
During the last couple of episodes of Season Five, everyone on The Other Side was able to see and communicate with each other, even Vampires and Werewolves. Eventually, everyone was able to physically effect the real world, but still not be seen by anyone.
In Home, The Other Side was destroyed, and everyone that resided on it, was either sucked into oblivion, or found peace of some form.
Lowering or dropping the Veil allows the inhabitants of the living world and the Other Side to interact. Destroying the Other Side completely, however, causes all ghosts to move on to the afterlife, either finding peace or being sucked into oblivion.