Each sport has it’s own home field advantage.  But not all games within a sport are created equal.  Below is a table that shows how home teams have fared for all MLB games since 2005.

Game TypeHome Team Win %
All Games54.2%
AL vs. AL53.8%
NL vs. NL54.3%
AL Divisional Game52.8%
NL Divisional Game53.6%
AL at home vs. NL60.3%
NL at home vs. AL50.4%

While some will argue that familiarity or being comfortable are keys to home field advantage, the main reason is the fact that the home team gets to bat last.  This is akin to having position in poker and being able to act last in the hand and having more information than the opponent.

The disparity for interleague games is likely attributed to roster construction.  American League teams go into the season with a designated hitter on their roster while most National League teams will end up using one of their bench players who is usually a much lesser player, thus giving the AL team an additional advantage over their NL brethren.