For starters: Every decision-maker who reads a contract has his or her own values, biases, intelligence, education, experience, etc. It's impossible to know in advance what those might be for a particular executive, judge, juror, etc. In persuading that individual to see things the way you want, a bit of extra language might make all the difference.
Plus, the future's often cloudy. It can be hard to know what difficulties the parties might encounter down the road. A less-terse contract might make it easier for the parties to collaborate effectively in dealing with unanticipated problems.
(Of course, many contracts are too long because their drafters didn't or couldn't take the time to be more concise, but there's usually a happy medium.)