I personally don't see astral projection as a part of magick. It's natural for our spirits, every person does it at some point when they sleep (out of body experience) at all ages. In harnessing the ability to astral project correctly, they experience for themselves energy, spirits, can interact with their spirit guide, and learn. By experiencing the astral or ethereal plane, they can begin to truly understand how it all works.
I don't discourage people from attempting to astral project. In order to do so, they must master meditation and trance, which takes quite a lot of practice and dedication. And we all know meditation and trance is the cornerstone of magick.
In projecting they aren't truly gaining anything physically. Wanting to astral project is usually tied to a desire to learn and see through the actual spirit world instead of wanting to bend the physical world around them like they're a wizard from a fiction movie. I find it more refreshing when someone asks about astral projection than desiring to cast a love spell, money spell, or be like Harry Potter. I think that those who do astral project have a firmer reality of what magick truly is, and what their limits are.
When is a good time to astral project? People who aren't even interested in magick (that I knew personally) used to practice it. My mother saw it as the ultimate way to let go of the mental hold on the physical, and look more into the spiritual side of life in general. It was sort of a small step to enlightenment in her eyes. To be able to let go of one's body while conscious is an excellent way to realize your mundane world is not the center of the universe or all that is. Some people have doubts that there is anything beyond the physical world we live in, and I have never met a person who astral projected fully who still had that doubt.
I began meditating when I was very young and I can tell you, I didn't truly know meditation until I learned to project at the age of fourteen. My mother actually taught me how to astral project, how to let go and enter a trance. Believe it or not, she made me listen to Pink Floyd's Breathe, and breathe to the beat, feel the music, and feel myself become the music, float away with it. It worked. I read Robert Bruce as a way to pick up any additional "tips" that could help more. And in practicing, when I entered my first trance, I met my animal spirit, and was able to see him clear as day. It was extremely encouraging, and I continued to practice until I made it. My animal spirit guided me. Learning to astral project was an experience that sticks out in my mind far stronger than any spell or ritual.
My personal experience influences my views I'm sure, but it is another perspect to take into consideration.