Every healthy human being lives on all ten levels of the continuum of existence, even though most of us are unaware, in waking consciousness, of levels more rarefied than levels 9 or 8. When you take the first actions to begin your magickal practice, you activate your conscious participation on level 9. The presence of your conscious attention there attracts the notice of the "natives": ghosts, dreams, nightmares, unresolved negative emotions, psychic vampires (who often have physical bodies--you know them by how they drain you), interdimensional alien monsters, primitive survival urges, Black Lodges, etc
When you do a thorough banishment, you ward out all such entities from your dedicated spacetime, while simultaneously shielding your neighbors from the energies you raise in your work and the effects such energies have on people and events. Banishments can be done to clean out and dedicate a house, apartment, office, automobile--any structure or place that needs it.
You can carry a protective field about you as you move through your daily life. After you've banished a spacetime for ritual, meditation, or any other purpose, and when the working is completed, imagine the boundaries of the space shrinking around you until it fits your form. This practice is useful in temporarily insulating your developing "extra-senses" from information overload, and in insulating your increasing energy from detection by undesirables.