Trial court practice and appellate court practice are two distinct skill sets. Trial court lawyers take and defend discovery, examine witnesses and handle evidence, and make tactical calls on how to build the record, while preserving issues for appeal that may or may not ever get argued. The appeal process is about "tuning" the law (as much as correcting trial court errors). Appellate lawyers are generally more steeped than trial court lawyers in the nuances of prior cases and precedent, so that they can bring out the finer points in the legal issues on appeal, and assist the appeals court in reaching the correct decision (advocating for their client, of course). Appellate practice is generally thought of as more cerebral. Trial court lawyers are often thought of as fighters with flair. These are gross generalizations, and there are many, many good trial lawyers who handle their own appeals, as well as appellate lawyers who appear in the trial courts.
In this case, the trial court practice wasn't so different than appellate practice, given the procedure as actually overseen by Judge Berman (i.e., there was no discovery, there was no evidentiary hearing, the record was set during arbitration). But Judge Berman could have gone in a different direction--as many were hoping for--permitting some (limited) discovery, and perhaps taking some live testimony. Kessler would have been the guy to handle all of that, and of course--knowing the arbitration record because he built it--was well positioned to argue the legal issues as well.
As Myt1 notes, it is very common to bring on another attorney for the appeal, who brings a deeper appellate court perspective to the briefing and experience in panel arguments. Typically you want someone with experience in the court where the appeal is pending, who may know the judges' interests and proclivities (so it's unlikely that Johnson's Supreme Court experience really means that much here). The cynic says that it doesn't hurt to add another billing code to the matter (as dcmissile notes, it's still in-house--just a partner of Kessler's). I'm sure Kessler would do just fine with the Second Circuit.
