One of the primary goals of NBA teams at the trade deadline is to get rid of their long-term salaries. There are many ways to do this, all based upon how good the player is who is attached to that long, expensive contract. Perhaps the most common mechanism to get this type of relief is to trade your expensive guy for someone equally expensive who is making the same amount of money on a shorter contract. That way, the light at the end of the tunnel is a little closer.
Under Ryan McDonough, the Suns never made trades like this.