I guess because of the crowd cheering the home team on ! Look at the Seattle Seahawks home games. The loudest stadium in the NFL. It helps the Home team want to win in front of the Home crowd. As far as importance? I think it's a morale issue.
Why do you think home field advantage is so important in sports?
Basically ...
1 - The "home team" does not have the stress of travel. Especially if they have to cross time zones.
2 - The "home team" fans tend to turn out in higher numbers, since they don't have to travel