Arnott only wanted to go to a playoff contending team. The Capitals are that team.
The Washington Capitals were pretty conservative during this year's trade deadline. They claimed Marco Sturm off of waivers a few days ago, but other than that were quiet—up until today. Today, they acquired defenseman Dennis Wideman from the Florida Panthers in exchange for a prospect and draft pick.
It looked like that was the only move they made, but then NHL.com announced the Capitals traded center David Steckel and a 2012 draft pick for New Jersey Devils' forward Jason Arnott (This has yet to be confirmed on the Capitals website though).
After these two trades, Capitals GM George McPhee seems to have acquired everything he was searching for this trade deadline. It was reported that all McPhee was looking for was a top defenseman and second-line forward. Between Wideman and Arnott, the Capitals have found both.
Additionally, Arnott has won a Stanley Cup, even scoring the cup-clinching goal for the New Jersey Devils. Arnott has vast experience that will be a great addition to the Washington Capitals.
However, one person alone cannot help a team win a cup. What Arnott can do though, is help the Capitals get the attitude required to win the cup. They've seemed to struggle with leadership this year—no one person seems to have stepped up to lead the team.
Arnott can change that. If he remains a Capital past this season, he could really be beneficial for the club.
Steckel will be missed though. He's known and beloved by Capitals fans for his contributions on faceoffs and the physical presence he brings. Arnott is similar in size though, so he will be similar in that respect too.
The Capitals' defense is still struggling with the injuries to Tom Poti and Mike Green, so I'm not sure they'll be able to overcome that to win the cup this year. However, if everyone gets healthy by playoff time, they will have made themselves a tough contender again.