Why?

You wouldn't be able to see any of the pitches

so the batter can see

the hitter should have a good view on the ball

In the old days there used to be seats there. But what used to happen was the team would send a spy there and read the signs the catcher was giving and tip off the hitter.

Look at yankee stadium they have a area with black seats. That used to be seating, but is now banned by MLB do to the above fact.

Also it is a distraction to the hitter to have moving objects in the background when he is trying to pick up the ball out of the pitchers hand.

So the guy up at bat can see the ball that's racing at him at 90 M.P.H., otherwise It's too hard to see a little white ball while 1000 people are wearing white and waving **** around, otherwise It can be too difficult just to keep your eye on that fast ball

to have a clear view of the bright white ball coming toward him.

It's a matter of safety and fair competitive balance.

This section of the park is known as the batters eye. Like everyone else is saying it is intended to give the hitter a good view of the pitch coming at him. The baseball has a tendancy to blend in with the fans if there is no batters eye.

Too far away from the action, and in direct line of fire.

It's considered a hitters backdrop so he can pick up the ball clearer after it leaves the pitchers hand. With people sitting in centerfield particularly wearing white shirts etc it would be difficult for the batter to pick up the ball and even potentially dangerous.

Last year the Red Sox let people sit in those seats. They handed out green T-shirts for the spectators to wear. This was suppose to keep the area dark enough for the batter to see the white ball coming out of the pitchers hand.
I'm not sure why they don't do this anymore.

The batter's line of sight (vision) is why. It's all about hitting HR's.

As stated, it's so the batter can see pitches. In addition to not being seats, you'll notice that stadiums generally have the area above the outfield wall there painted a solid dark color, so that there is more contrast against the white of the ball. This is usually called "the batter's eye".

Basically so the hitter could c the ball leaving the pitchers hand. Its called a batters backdrop. It must be black/ green any color but white or gray.

It is considered the hitters' view. Nothing may obstruct the hitters view!