It serves as the "batter's eye". Just something to give the hitter a solid color background when hitting.
There's no historical purpose for it that I know of. Just the way it was designed. As to additional seating, Ameriquest Field (I sure hate that name

, The Ballpark In Arlington was so much better

) seats over 50,000 as it stands right now. That's a lot of tickets and revenue seems to be very good right now (plenty of ticket sales). Good tickets are almost always availiable for every game and it's not like the stadium's empty or anything. Tons of people at every game.
Guess that's what frustrates me about them not spending money on a solid #1 pitcher. The revenue is there to do it, so why keep cutting payroll?
Anyway, hope that helps some.

Yes doctor, I am sick.
Sick of those who are spineless.
Sick of those who feel self-entitled.
Sick of those who are hypocrites.
Yes doctor, an army is forming.
Yes doctor, there will be a war.
Yes doctor, there will be blood.....