The reasons for such a move are numerous.
Let's start with the fact that Dallas needs a new free safety in 2006 that can handle himself in coverage situations deep downfield. As we saw in 2005, Keith Davis can handle himself, but Dallas needs a free safety with better coverage skills, especially late in the fourth quarter with the game on the line.
Let's also start with the fact that Coleman was with Bill Parcells in New York. In fact, the Jets drafted Coleman in 1996 out of Texas Tech. Thus, the Cowboys head coach is already familiar with his game and would likely approve, if not initiate his acquisition.
Coleman is also a "cover guy" who has decent size (6-2, 206).
Strong safety Roy Williams is better suited to patrol the line of scrimmage and make bone-jarring hits. He needs a player to compliment those skills and Coleman certainly fits the bill.
Coleman is also a 10-year veteran, meaning he has considerable experience in the league and should not command a huge salary- which makes more obtainable for the guy writing the checks- Cowboys owner Jerry Jones.
Let's end with the fact that Coleman shares the same agent as Parcells, Jimmy Sexton. The Sexton-Parcells combination, of course, has worked magic in year's past in free agency and will surely do it again.
Last year for the Texans, Coleman played in 15 games, recording 52 tackles and one interception.
He was benched late last season after missing a walk-through practice and rarely played after that. For his career, he has 25 career interceptions with 429 tackles.