>>12482803There's no definitive theory. I remember in a psych elective I took they taught us that it's probably a rehearsal process and used to encode memories/lower-brain functions. Or maybe it's our consciousness projecting into the astral plane; both can be true I guess.
>>12483454You can train yourself to remember them. I keep a sporadic dream journal, but I used to record them every morning. I could tell you every dream I had the last week and I still visually remember most of the ones I wrote down when I look back on them. I've also lucid dreamed but never because of the dream journaling.