What exactly does removedOnCompletion = NO do?

If removedOnCompletion is YES, the animation will be removed on completion and there is nothing to "fill forward" at all. So if you want to use fillMode you must set removedOnCompletion to NO.

removedOnCompletion:

Basically, is the animation removed from the layer's actions when that animation is done.

YES - If you intend to use the animation only once, you do not need to use the delegate method to remove the animation from the layer upon completion.

fillMode:

This property controls what happens visually when the animation is completed, depending on the type of fill mode specified.