The Right Frame for the Right Picture
Look at the two pictures below. Which frame is more fitting for the picture? Why?
Personally, I like the look of the frame on the top with the tone and flourish of the structure, while the minimal style of the bottom frame seems a bit too contrasting. This is very much like aiming for a more classically-inspired orchestral score (top) vs. a New Age-feeling orchestral score. Consider the score for Civilization IV & V- many leader songs are derivative of national songs for each country and the entire Civilization IV underscore consists of Medieval, Baroque, Classical, and Romantic repertoire (minus the modern era, which features music by John Adams, a contemporary composer). That's what you see on the top.
Music, like a picture frame, or for another analogy, like the finish on a piece of wood, should emphasize the good aspects of the game and help mitigate focus on the not so great elements. It does this in two ways:
1. Immersion- by subtly helping to draw the player in (generally the underscore), music can gently nudge the player's emotions and mindset. For example, fast, urgent music might cue a player to focus more during a battle while slow, gentle music might relax them while walking through a peaceful setting.
2. Melodic Interest- game music can also generate its own emotional value such as main themes or melodies for particular characters/players/environments (generally the overscore). These themes become memorable if they are well executed and catchy enough, and often can shift the player's mindset to a more nostalgic side, reminding them of another part of the game or clearly cuing them to the approach of a certain character or environment.
Think of the immersion as the wash behind the painting- the broad, vivid colors that define the basic shapes (like the sea). Melodies are the focused points which appear here and there, building connections and defining things "that's the bad guy!" (like islands in said sea).
Underscore is music that is more felt than heard. This is the music you hear in a battle scene, while the character is walking, in a village, etc. It is often in a loop, with a nearly impossible to find transition back to the start somewhere.
Examples: A B C
Overscore is the music that is heard more than felt. This is the main title screen music, or special music in certain instances where a melody is present, or perhaps a song that plays on the radio in the game. It is often in the form of a song, with a clear ending and beginning.
Examples: A B C
Dividing the Question
Some games have multiple composers, sometimes with one tackling a certain genre or style and another tackling another. Positive aspects to having multiple composers is that a large score can be completed faster and that you can have more distinction between different parts of the music. Downsides are that the music may not be as clean in transitions and some pieces may have a different clashing quality to them. Normally, if there is over 20 minutes of music or the composer(s) are busy and you are asking for low cost music, dividing the job is best.