[Outer before inner walls disabled make gap stuck]

My question may be too open. So What I want to know are

- When the use the Outer before inner wall?

- Why the Outer before inner wall disabled as default if it make dimensional accuracy?

 

[Outer before inner walls disabled make gap stuck]

I printed Piranhaz using Cura 4.2.1 Linux and Ender3, however, the model teeth stuck together and it cannot be moved. In the model, upper and lower teeth are quite close but not attached. Then I try several setting and found that enable the "Outer before inner wall" can fix the issue.

 

The picture is the teeth-stuck fish, i have to force them to open using screwdriver.

 

Why the Outer before inner wall option can make such the difference?