#2 Griezmann provides a boxload of tactical options

To compete for all the trophies, Barcelona need to have more than just one tactical setup, which seemingly is the case right now. Erik Ten Hag has two different setups for Eredivisie and the Champions League, and Barcelona need something like that too.
Griezmann could himself cover all three positions in the forward line and also the additional position of the second-striker. With him, Barcelona can have three different setups in a 4-3-3. The most popular being Griezmann-Suarez-Messi, in a 4-3-3, but it can have a lot of varieties, with Dembele playing down the left and Griezmann in the middle in the same formation.
If required, Barcelona can also move Messi into the attacking midfield spot and accommodate Dembele-Griezmann-Suarez in the forward line. If Barcelona want to try out a 4-4-2, they can do so with Dembele or Coutinho as wide midfielders on the left and Messi on the right. Or they can have Messi as a false 9 in a 4-3-3 due to Griezmann's comfort while playing down the middle. There can also be a 4-2-3-1 formation with Messi as CAM with Suarez as a striker, or Coutinho as CAM with Griezmann as a striker.
A diamond formation, 4-3-1-2, with Messi as the tip of the diamond, is also a very lethal tactic.
Need more?