That可以引导表语从句,但原句the biggest disadvantage of advertisement is that under their influences.中的under their influences不是一个完整的句子,原句少了that就是一句很好的句子.
哦,原来如此,楼主就不要断章取义嘛,the biggest disadvantage of advertisement is that under their influences,some pople may follow the fashion blindly .这是一句完整的句子
,that在句中起联接副词的作用