Trading in the draft can be beneficial for a team. Depending on what their need is, they can either trade up or trade down to address that need with the proper valued pick. This is what the Colts did last year with the third overall pick, trading down to sixth to get Quenton Nelson.
In that instance, the Colts got multiple other picks back from the Jets. That is what Chris Ballard wanted to do in order to accumulate talent. He wanted to have as many picks as possible. Doing that helped the value of the picks increase, and he drafted well with them.