There are many reasons to shield. When practicing the craft or even just exercising your psychic gifts you will come across those who will drain your energy or unintentionally send out harmful energies.
Emotions are a powerful energy. Anger and Fear are two of the most powerful emotions. That wave of negative energy projected out from a person can effect or drain you.
Sickness.. When someone is sick, they can unknowingly drain someone close to them. They don't intend on doing so. It just happens.
Magick attack. Well, this is very rarely done. But it is also a possibility. When we shield ourselves properly, it protects us from others harming via magick.
Exercise in Shielding-
When someone surrounds themselves in white light without grounding they run the mistake of doing what I call "Flaring the Aura" This is not a good thing. Not only does it use your own energies to "power" the shield around you but makes for a bright target on the astral planes for any nasties that are drawn to bright beings. If anything hits your shield you will be drained anyway in the reflection of the attack because you used your own energies to power it. In other words.... not good at all!
1. Do the grounding exercise above.
2. After you have completed the grounding exercise do not let go of the earth.
3. See yourself in a dark room.
4. Hold your hands out and visualize a bright ball of energy drawn up from the earth. Do not discard it if you don't get white. Each person's energy that they draw from the earth is different and resonates with them. Allow it to grow in your hands till it is too heavy to hold. Let it drop and grow till it is big enough to walk into.
5. Walk into the energy. Feel it surround you.
6. See it seal around you and become an unbreakable sphere around you.
7. Come back to yourself refreshed.