It’s true that these two phrases have rather similar meanings, but if we look closer, we’ll see a few subtle differences.
To let something or someone be is to leave that thing or person alone.
Let sth/sb be
Joel: Did you hear what that guy called me? I should go and beat him in the—
To let go of something is to stop something or someone from bothering you.
Let sth/sb go
Joel: I can’t believe that I didn’t get the promotion! I just … can’t.
向生硬的文法規則說 bye bye