When on Earth, spirits can choose to take many forms to interact with those with the Sight. Sometimes, this may be their true form, in all of its detail and glory. But many times, they choose to present in simpler forms, such as orbs or wisps of light.
In my personal conversations with spirits, these simpler forms are often a means of saving energy.
And I realized, when astral travelling, could humans do the same?
I cannot think of any reasons why not. It would definitely be an energy-saving measure for us too. One of the reasons beginners struggle with astral travel is because of the energy requirement it takes to leave the body while sustaining the form. It could, potentially, be easier for beginners to start astralling as orbs or wisps as well…
The only downside is that it may be harder to interact directly with the world you astral to. However if you are travelling only to observe, or for simple conversations with spirits you already know, using a wisp form would be a great compromise to ease the energetic strain of astralling.
The Malachite Library 