I can think of three reasons why introductions need to be looked at. First one would be that the character is important, but doesn't make enough of a first impression. The reader's not going to get involved with a character if he/she forgets the character within a few paragraphs of getting introduced. So let your character's inner stars shine from the start. Or at least a little bit to entice the reader.
Then there's the fact that a character wasn't really defined when you wrote him/her at first. It's perfectly natural, because it takes time to get to know a new character. Time that can only be taken by writing the story.
Also, in the process of writing a draft, you'll very possibly find that the character veers off in another direction as the story goes. Because of that, the original introduction and who you discover the character to be, won't coincide.
So when it's time to look at your characters' introductions: look for traits that you want the reader to know from the start. Does the introduction of every character show those traits in a way that imprints the character in the reader's thoughts? If not, you'll have to rewrite the scene to create the impression you wanted.
There's only one chance to create a first impression for a character. Make it count.
Look Out for These:
1) The character seeming like different people between the first and later parts of the story.
2) An introduction that isn't memorable, despite the fact that the character is supposed to be.
3) Readers not engaging with an important character, or failing to remember that they've been introduced.
What do you do to get the introduction of your characters just right?