Because I guarantee you, if you came to England and you spoke to a native speaker, it's quite likely that you would hear on informal contraction in conversation because some people, like I said, they do use them a lot, so it's good to know them on if you want to use them.
因為我向你保證,如果你來到英國,和一個以英語為母語的人說話,你很可能會在對話中聽到非正式的收縮詞,因為有些人,就像我說的,他們確實經常使用它們,所以如果你想使用它們,瞭解它們是很好的。