Its hugely important....if CA never saw it, and it did not exist in the Anthony household where GA resides, then how in the heck could he dispose Caylee in it? He couldnt.
Also, it shows that ICA did have the shirt, with her or at a friends or in the car. OR perhaps she bought the shirt knowing she was going to kill Caylee...remember her claim that "zanny" had clothes for Caylee at her house. So, she bought it, dressed Caylee in it, in preperation for the Nanny story.
Further, CA would not see the shirt if it never existed prior to the time Caylee went missing...why are people having a hard time with this?