When manga first started to be published, all of my friends who loved manga but didn't know Japanese wanted to be rewriters. It was a way to be a part of it without having to take the years necessary to learn Japanese.

Nowadays I'm not quite sure if they'd be so enthusiastic. Very few people can see what a rewriter does for a manga, and if any of the dialog isn't quite how the reader imagined it, the rewriter is one of the first to be blamed. In other words, the rewriter is the last to be praised for a job well done, and the first to be criticized. Not the best position in the world.

The most vital domain of the rewriter is characterization. If the characters sound like the characters, then the rewriter is doing a good job. If all the characters sound the same, then there's a problem. Apart from the one-or-two-word lines of dialog, English provides a wealth of ways to differentiate a one character's voice from another. Let's take an example and put it into a variety of different voices.

Let's take the line:
I'm going out with friends. Do you want to come along?

Tough Guy:
Me and the guys are goin' out. Comin'?

Child:
I gonna meet up with some friends! You coming too?

Young man:
Some of the guys and I are hitting the bars. You coming with?

Young woman:
A few of us are getting together. Do you think you can come along?

Elderly:
Several of us are going to paint the town. Do you feel like joining in?

Polite:
A few of us had plans to go out. I do hope you can accompany us.

Affected:
Plans are afoot for a group outing. Your presence is requested.

Evil
I am leading my minions out to fulfill my plan! Join us...or die!

Okay, the last one wasn't exactly true to the spirit of the original line, but you get the idea. All of the above sentences mean pretty much the same thing, and the only thing that distinguishes them is vocabulary, and that's the tool of the rewriter/adaptor. It's possible for a manga to use the first, neutral sentence in the translation, but the experience wouldn't be as rich as is could be, and worse, wouldn't be as rich as it was for Japanese readers when they read the original book.

Since rewriters/adaptors are usually the professional writers of the team, they also have to keep their eyes open for foreshadowing, dramatic repetition of words or phrases, comedic repetition, and other tricks of writing so that the book does what the original author wanted it to do.

There are some translators who do both translation and rewrite, and I count myself as one of those. The problem there is that these are two distinct jobs that need two very different skill sets. I was submitting short stories to magazines before I ever tried to learn Japanese, so I had some background in writing that I could bring to translation. Other translators I know learned how to do adaptation while on the job of doing literal translation. But basically a translator/adaptor is one person doing two jobs. Because I can translate and adapt at the same time, it doesn't take me quite as much time as it would take two people, but it takes me longer to do the full job than it does to do a literal.

And there are many rewriters that I respect greatly. I was fortunate enough to edit several manga scripts rewritten by Gerard Jones, and those scripts were just about perfect. Although he didn't know Japanese, Gerard's knack of capturing the voice while keeping the information exactly the same always amazed me. And Fred Burke was a go-to guy when I was at Viz for scripts that involved mysticism. He knew how to capture that tone like few others in the business. I'd list more, but I don't want this to sound like an acknowledgements section.

Although the number of translators who can also be adaptors is growing, it isn't anywhere near enough to cover all of the manga that is being produced, so splitting the duties into the two skill sets makes sense. The rewriters are an extremely valuable asset to translated manga and anime, even though very few people can distinguish what they do.