When you are driving a car with a manual gearbox, it is necessary to change gears so you will have enough power from the engine and cruise at your desired speed. You just cannot stick to a single gear since your car will be ineffective and you will have some problems along the way.
Driving your car is similar to how you play the game of poker. It is a good practice to change gears when playing poker and later your game. Varying your style of play will help you adapt to the dynamics of the game.
Changing gears means you trying to be unpredictable by deviating from your usual kind of game. Remember that your poker strategy of the start of the game might work but as you go along, it may give you some trouble.
Shifting gears during a poker game does not mean going from being a super tight player to an ultra loose one the next second. You should vary your game depending on how the poker game is going. The other players and how things are going for you may require you from playing it loose to switcing to being tight.
There are several reasons why you should change gears in poker. The other players around the table will somehow figure out your poker strategy and game style and this will mean a big advantage for them.
If you go tight-aggressive all the way, there will come a point when your opponents can push you back and take your chips away. Vary your game so you will be unpredictable. Tight aggressive is a good gear to stay on but you need to change gears in a number of situations to keep the other players guessing.