How important are openings?

I have a FIDE tournament in a few days. I'm completely confident in the middle game. Everyone says that opening study should be reserved for when I'm higher rated (~2000).

This is true, because by following general strategy, I can transition into the middle game easily. With a plan, it's hard (if not impossible) to go wrong.

Why, then, does everyone advise to review openings before a tournament? (I have taken perhaps a brief look at the classical variation of the Ruy Lopez, but that's it.)

People are obsessed with openings, just the way it is. I guess just play what you normally play, something which gets you into a playable middlegame.

"It is important for club players to build up a suitable opening repertoire." - GM Artur Yusupov (2010)

Some people claim to do well without opening study. In a few days, you will discover the degree to which you fit into that category. In the meantime, a few days is probably not sufficient to do much about your current opening knowledge.

In general, most club-level players are out-of-the-book after around 6 moves or so, so memorizing long variations is not necessary.  But knowing what strategy works for an opening can help.

If you become familiar and comfortable with the ideas behind a few openings and play them often enough, you are less likely to fall for opening traps and more likely to get a feeling for what kind of middlegame strategy works.  For example, the Bishop's Opening and Vienna Game often try to get in a early pawn to f4 and K-side attack is the result, potentially aided by castling Q-side.  The Caro-Kann and Slav Defenses try to get the QB outside the pawn chain before ...e6 locks it in.  in advance variations you get a feel by frequently playing it for when to attack your opponent's pawn chain to open up the game.