It could certainly be questioned on whether or not the Twins truly need another catcher on their roster. With Mitch Garver really needing an opportunity to prove he can be an everyday catcher and Jason Castro on his way back from injury the catcher position may be full. Much like Harper earlier in this post, Yasmani Grandal is good enough of a talent that you add him and figure the rest out later.
The question facing the Twins in 2018 as their roster currently sits, assuming everyone is healthy, is do they go with the bat of Garver or glove of Castro for their starting catcher. A Grandal signing would allow the Twins to really send out a good glove catcher who also is pretty good with the bat for a catcher.
When it comes to defense he ranked at the top of the list in several pitch framing metrics. Looking at what Statcorner has compiled, Grandal was able to convert 9.4% of pitches he caught outside the strike zone into strikes. In his limited action Castro topped that list while Garver finished at 6.4%. Also according to Statcorner that resulted in a swing of 104 calls that were called in the Dodgers favor over 2018.
When we add to those defensive numbers the fact that Grandal is a career .240/.341/.441 hitter it shows just how well-rounded of a catcher he is. In the past three seasons Grandal has also shown good home run power hitting 27, 22, and 24 home runs over those seasons. So while this may not feel like a screaming need to add a catcher it would be a significant upgrade to what is currently on paper.