No-one is really sure what the actual purpose of dreams is, there's no general consensus, but tons of theories. What you mentioned is one theory of many, one which I don't personally think is true. If we are simply sorting through memories, then why do we dream of places, people and situations we've never seen? For me anyway, it's rare that I relive a memory in my dreams.
Instead of "going through memories", I'm more inclined to think that a purpose of dreams is to sort out unresolved waking issues, rather than memories. For example, persistent nightmares can often be linked to a waking problem- one which your subconscious is trying to draw your attention to. In this case, lucid dreaming can help you get more in tune with your subconscious. You can consciously work through whatever feeling/emotion/issue your dream is showing you, and hopefully work towards resolving it.
Another thing I want to throw in is back on the topic of memories. In my experience, lucid dreaming has caused me the opposite of memory loss. It has made me
much more aware of my surroundings, and has made me pay much more attention to small things in an effort to understand the relationship between dreams and waking life.
So in conclusion, don't worry about becoming forgetful (or indeed experiencing any other undesirable side-effect) due to lucid dreaming! I hope this has addressed your concerns, and good luck with your lucidity!

Any other questions, feel free to ask.
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