Headcanon:

Halfas astral project as a defense mechanism if their physical bodies become too injured to continue a fight.

A halfa's physical body in all 3 forms (human, ghost, and mid-morph) is far more durable than a normal human, but isn't immune to damage. They are able to survive many injuries incompatible with human life due to the nature of ectoplasm and their natural healing abilities. Destruction of the physical body beyond its ability to heal or crushing of the ghost core are both ways to kill a halfa. Unfortunately, the physical body of a halfa has limits that vary based on their power level. Once one of these limits (usually physical exhaustion, being knocked out, or severe injury) are reached, the halfa will automatically be tossed out of their body in ghost form and the body will fall unconscious and revert to human form.

This allows them to continue fighting (protecting both their core and their body from further damage) in a situation where they would have been vulnerable. The downside to this is that the astral form of a halfa is weaker than the full halfa due to the body still requiring some of the ectoplasmic energy the core produces. Healing of the physical body is also slowed by this. Not as slow as normal human healing, but far slower than their normal speed. They also cannot travel too far from their body as they are linked. (Maybe like half a block max range?)

Depending on the halfa and the situation, they may either return to their body as soon as the danger has passed/fight has ended, continue projecting to protect their body from danger as it recovers, or split their time between sleeping and projecting in cases where the body will be mostly put of commission for awhile.