It is an additional challenge to see if you can master the crop before it is no longer available. Bragging rights for thoes who have the time to get the sign post as no other will have it in the future. Same as limited time items etc.
to also prove to others you have no life
And from what I can tell, they arent temporary if you get the mastery. Cause I still have them available. And they were supposed to disappear two days ago.