It's useful knowing several languages.为何用knowing不用to know

knowing :动名词,表示已经存在或者已知的事情.
to know ,表示要做的,未发生的.
It's useful knowing several languages.懂多种语言是很有用的.（已经懂多种语言）
It's useful to know several languages. （如果能）懂多种语言是很有用的.（不确认已经懂多种语言）