Creating a party is quite different from just creating a character. When assembling a party, the group of players must themselves influence one another's choices. A party must not be thought of as merely a collection of characters, but a complete and well thought-out unit.
Players who all sit together and brainstorm character ideas will often find themselves seeing niche rolls which they themselves would like to fill. Such an environment also lets the players more fully voice their concerns or needs for the group.
You will find groups with players who all want to be the same thing. By creating characters together, and allowing every other player to see your own rolls, your ideas, the entirety of your character, there becomes an air of understanding. When each player creates a character in a closed environment, they do not fully appreciate the abilities of the other characters. When creating all at once, everyone can see the party take shape.
Voicing worries will allow the group to say things like "well, we're all fighters.. but we're going to get ourselves hurt.. we need a healer!" Such comments will shape the party.. maybe someone will take some healing skills, or someone will create a warrior-priest.
If the group does their Chinese portrait together, after doing some basic character brainstorming, more ideas can be passed around to make each character more full.
By brainstorming "origin stories" of all the characters, the players can choose to align their histories with one another more easily.