Say, you apply transform as an effect and make a bunch of rotated copies of an object. Now those copies are an effect and if you center the object it will only center the original object and not the entire thing you are seeing.
How can you “flatten” the object so instead of a transform effect you have an actual object with no effect that looks like the original object with the transform effect?
Use Object > Expand Appearance on the transformed ‘group’. You might need to Object > Ungroup the result.