Really he wasn't. I think Turner's value has been wildly overrated by some. He could do some unique things, but also couldn't do many of the things you want/need your 2/3 to do.
Kevin Pelton on ESPN seems to think the Blazers got a terrible deal, and points out some of his flaws ...
For the Blazers, Turner was plan B after Parsons spurned their post-midnight advances to sign instead with the Grizzlies. While Parsons' fit in Portland was easy to see, at least this offseason, Turner is a much more curious fit. He's a player who requires the ball in his hands to succeed joining two guards (Damian Lillard and C.J. McCollum) who are both superior shot creators.
According to Spor**** tracking data on NBA.com/Stats, Turner's time of possession with the ball was about 32.3 percent of the time he was on offense. By contrast, no Blazers regular besides Lillard (48.8 percent) and McCollum (29.9 percent) possessed the ball more than 10.8 percent of the time. Portland's wings generally spotted up in the corners and kept the ball moving in Terry Stotts' motion-based offense, very different from Turner's role in Boston.
The issue isn't quite as bad if Turner comes off the bench rather than starting at small forward alongside both Lillard and McCollum, but even then Stotts' rotations kept one of the guards on the court at any given time, so Turner would always be playing with another primary ballhandler.
It's worth pausing to note here that putting the ball in Turner's hands gives an offense a limited ceiling. For all the excitement about how he played with the Celtics, last season's TS (.513, the very best of Turner's career) still ranked 97th of the 117 players who saw at least 2,000 minutes of action last season. And Boston scored 4.9 fewer points per 100 possessions with Turner on the court, according to NBA.com/Stats.
Still, having Turner run the offense is probably a better option than having him spotting up because Turner is such a poor outside shooter. He made 20 3-pointers all season at a 24.1 percent clip, and while Turner is more accurate inside the arc, his slow release allows defenders to help over him. In my parlance, Turner has little gravity as a shooter, creating problems for Lillard and McCollum.
All of this might be worth it if Turner were a defensive upgrade. He improved at that end under Brad Stevens and was playable in the playoffs. Still, Turner isn't much better than average, and I certainly wouldn't call him an upgrade over Maurice Harkless.
Add it up and the Turner signing looks problematic before we even get to the money, which pays Turner like a player who creates 3-4 wins above replacement per season. (I actually project him a little below replacement over the next three seasons.) Portland threw most of its cap space at Turner -- all of it, in fact, before the team starts renouncing free agents who were part of the team's rotation last season.