Its a literal place where three roads cross, a lot of spells mention them due to various connections to energies of things that relate to it, such as fate , death, change, a choice, the Goddess Hecate, etc. A lot of spells mention them because for some workings these energies can be an assistant to the magic, I can't really go into detail because I don't know what your specifically asking.
A crossroads is, like was previously stated, any place where any two or more differing routes (paths, roads, trails, etc) meet/cross. Quite literally, a crossing of roads.
As for what they are used for, and why, it centers around what they represent. Things coming together, things going their separate ways/splitting apart, finding or losing direction, and of course journeys in general.
For example when removing someone or something from your presence, carry the item representing/containing the other party to a crossroads and leave it behind. The idea being it is now harder for the entity/person to find their way back to you because there are different directions to choose. You literally get them lost and unable to find their way back.
Or conversely, if you want to bring something to you (like a guide or other entity) a crossroads can be a good place to perform the ritual/casting. You have a physical representation of calling out in all directions, and of having a path the entity can follow easily to reach you regardless of what 'direction' they might be in.
Thank you! I had originally thought that it was related to astral projection (like a crossroad between the physical and astral realms), so this helps me out a lot!
I suppose there are similar things to crossroads as they might relate to astral projection/travel. Though, I find people look for things that relate more to the ideas of veils, borders, or portals.
For example 'between' times like dawn or dusk, where it is neither day or night but instead something between the two. Or times of heavy mist or fog for the same reason.
Mirrors (natural and man-made), cave entrances, still pools of water, wells, bridges (particularly of stone), tunnels, and other doorways/entryways have also been used or meditated upon as well. Many of these things are associated with transitions to other worlds and spiritual journeys- both outward, and inward. So it makes sense they would be useful in more modern methods as well.
