You are plotting the story. You write down what will happen, what problems will arise, what obstacles you will place so the character won't reach his goals immediately, what he's going to do to overcome these problems, etc.
So all these things will be happening to your character since it's his story we are telling.
Does it make sense then to have your character in mind as you plot these things?
It does. Because it will depend, based on the type of person he is, how he will face these problems, what he will feel, what he will think, what he will do.
Different types of personalities make up our world. Some people worry too much, so whatever problems come along, they will worry with the same intensity. Other people view the lighter side of life. Minor problems do not affect them as largely as major problems. To some challengers are welcome -- they thrive on them. To others, challengers are viewed with fear and uncertainty.
As you're plotting the events of your story they have to correspond with the type of personality your character possesses.
What happens when you plot without thinking of your character?
He will act ‘out of character.' He will do, say, think, feel things that don't suit his personality.
If your character is a worrier and you place him in a situation where he doesn't worry, then that's making him act according to how you want him to act in your plot.
You're manipulating him to suit your plot -- You're not writing with his personality in mind.
As you plot the events in the story, simultaneously build your character.
Cross-reference what you have written about your character and the situation he is in. Do they correspond?