I have a personal hypothesis, born out of studies I read a long time ago and haven’t kept up with nor really bothered to research more (so take it with a grain of salt), that dreams are two things happening at once:

•Your brain organizing your memories of everything that happened that day, including every thought you had even if it doesn’t have a physical event attached to it.

•Your imagination adding as much of a cohesive story as it can to those often times unrelated memories.

I always picture it like still images that change rapidly one after the other, sort of like flipbooks, and then your “conscious” mind trying to keep up with it, finding no logic, and creating a storyline instead.

I’ve found myself lucid dreaming before, and despite being in control and knowing it’s a dream, I’m still asleep, so I end up making dumb choices or playing along with my dream.

The dreams I remember tend to be strangest/goofiest ones or the ones that had some emotional impact on me. However, when I analyze them while awake, I realize that there was a lot of extra “content” that I didn’t add or doesn’t fit into the dream. Like how somehow the place and the people I’m with change every “scene”.

Sometimes I wake up with a phrase resonating inside my head, with that feeling you get in your mouth when tou want to say something. And since I’m bilingual, I’ve had dreams with both languages happening at once. Hell, I’ve even had dreams where I’m speaking Japanese “fluently” (i.e. it feels fluent in the dream but I know it must be gibberish, since I don’t speak the language).

Sometimes they help me face subconscious anxieties, sometimes they give me solutions to problems I’m having IRL, but more often than not, it’s like I’m watching the randomest movie ever. And I do think they’re a “window or the subconscious” but not in the sense I think you’re asking. Since they’re memories and imagination, it is your subconscious that is choosing to focus on specific aspects or the storyline you create. So, analyzing them can help to see what’s going inside that blob of fat we call brain.

Tl;dr: they feel like when you’re fantasizing/daydreaming but a lot less cohesive, and can be helpful every now and then.

I don’t know how dreams happen to people with aphantasia, and I know my explanation would be wildly different for them, but that’s how I see dreams.