Summary
Birch is a solid backup center, and certainly has his fans among the league. He has nice touch around the rim, and can hit floaters as the roll man in the pick-and-roll. He's started to show an ability to stretch the floor and hit a few corner threes, especially since joining the Raptors. He's also a good offensive rebounder, finishing in the 70th percentile or better for his position in offensive rebounding rate each of his four seasons in the league, according to Cleaning the Glass.
 
On defense, Birch is solid but not a rim protector that offenses fear. He can move decently on the perimeter but isn't someone you want out there guarding perimeter plays off switches very often. While Birch is a solid backup center, that position is the easiest to fill in today's NBA, so he may not be able to get much more than the minimum, if he's even able to get more.

Cap Considerations
Birch will be an unrestricted free agent with a $1.7 million cap hold and Non-Bird Rights. This means that the highest starting salary the Raptors could offer him using the Non-Bird Exception is $2.1 million. This should be enough to retain Birch, but there are scenarios where it may not be. Alternatively, the Raptors could use their Mid-Level Exception or Bi-Annual Exception to re-sign Birch if he demands more than $2.1 million, or use cap space if they operate under the cap.
 
Other teams will have multiple ways to sign Birch as even teams without cap space could possibly use any of the Non-Taxpayer MLE, Taxpayer MLE, or possibly even the Room MLE or Bi-Annual Exception to sign him. There's also a chance he could be had on a minimum contract.