Automatic app updates
Texts not deleted
That's just a small list of ways you can eat up storage (I'm assuming you mean that and not RAM memory, which is designed to be full) without realizing it. You'll need to go through your storage settings to see what's there.
Side note: If this is mostly a cache issue, you'll be best to leave it alone if it's not causing any problems. If you delete it, apps will simply rebuild it because they are designed and work most efficiently with some cache build up.