Beginner & Intermediate Level: Game analysis: Opening Theory, Opening Understanding
Centuries ago, when chess books first appeared, most chess fans memorized the masters' movements at the beginning of the game. The game was too complicated, then better to follow the masters' examples.
This hasn't changed that much. Most follow the fashion trends because if GMs play those, there has to be a reason. Actually, there are several reasons behind a GM selecting a specific order of movements in the opening, and those reasons are more important than memorizing the moves themselves, regardless of the player's level.
Chess is about precision when solving the problems on the board.
If someone studies an opening in order to memorize the moves, then the solutions from his moves will be no different from a +2600 player (assuming his opening book is a good up–to–date one); until the book suggestions end, his memory fades... or the opponent takes him out of the book.
Now, the reason why masters spend time studying openings, is because the variety and possibility of different plans. As the pawn structures can be modified several times within a few movements, so is the piece activity, leading to some plans being effective against a specific pawn structure and piece disposition, and –sometimes– ineffective because a slight different piece disposition or even a simple pawn move.
Then, rather than "eating" the clock –during the game– working on the possibility and precision of a plan (or different plans), they do their homework prior to the game. When memorizing the opening's theory, amateur players see the results from such work; but, without doing the job themselves, they lack the necessary understanding to play with enough precision when things are slightly different from what they remember.
Studying an opening system is studying the plans in that system.
In essence, an opening's plan is an effective way to arrange the pawns and pieces, to create threats against the opponent's position (or to stop the opponent's). If an amateur follows a GM's example, but don't know nor understands how those threats come to be, then –no wonder– he will likely miss them or lack precision when trying to solve the problems on the board.
Now, not all threats are the same. Some may be simple tactics and others may go around playing a Rook's ending with a clear initiative at hand. Whatever they are, choosing an opening system implies you have to study them: How to use them and how to defend against them.
It's only at this stage that memory comes into play, when you remember the work you've already done.
The following game comes from an analysis request in forums. It's a 30+30 game, and Black player posted it asking for ideas to finish his kingside attack. He wasn't aware of his weak previous play. This is the main reason why people should try to play with stronger players as, sooner or later, the defects in their game will be exposed. And that's how we improve in chess: By working on removing the defects in our play.