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If your builder function contains a subtree that does not depend on the animation, it's more efficient to build that subtree once instead of rebuilding it on every animation tick.
If you pass the pre-built subtree as the child parameter, the AnimatedBuilder will pass it back to your builder function so that you can incorporate it into your build.
Using this pre-built child is entirely optional, but can improve performance significantly in some cases and is therefore a good practice.