Hi there, I apologize in advance for the lengthy response regarding the altar. I can only state & explain what I know based on my experiences as well as through the knowledge gained through some of the resources available on BALG. The others have already given a pretty all around explaination however here’s my take.
The altar works as a catalyst and stoarge point for the energies you are calling into your temple and on the altar. To note depending on which energies you are generally working with, (demonic, angelic, planetary, necromantic, etc.) and what types of magick you do the altar will absorb, store it, transform it and transmute and emit those energies to the items placed on the altar as well as into the immediate environment around you. You can even feel the energies you have called into the altar by closing your eyes and placing your hands on it. However note if you really want a strong and useful altar remember to consecrate it. ways to do that are available or explained on here or to get a headstart E.A koettings Mastering Black Magick Course should give you a headstart.
NOTE: It’s a really good idea, which was something that was imparted to me, to create a new altar for every new system of magick you are working with, if you don’t - the energies of a let’s say a black magick altar may not necessarily work all that well most of the time with spellcasting that focuses on angelic and healing forces, that being said you can still do powerful healing spells by using black magick.
Astral projection or mostly refered to on here as soul travel (there’s a difference between the two), you can and should use soul travel to help yu out in your work with magick, it’s one of the best tools in your toolbox. With it you can travel to spirits and be taught directly by them, visit loved one’s for example without needing to visit them physically, through soul travel honesty anything is pretty much possible.