动词ing开头为什么要用has 比如working at home has some advantages.

从英语语法理论上讲,动名词(即动词的ing形式）作主语算第三人称单数.如果谓语是有或完成之类就用has.
如：Playing football is my hobby.
Planning so far ahead has some disadvantages.
have当"有"讲,主语是第三人称单数用has,其他人称、数均用have.
have用于完成时也是如此.
如：I have a brother,but my wife hasn't.
I have been to Beijing twice,but she hasn't.