yes. and even in english there is a subtle difference. "this desk is big" describes the desk as big. "this is a big desk" identifies what "this" is. but of course, in english, these subtle differences don't really matter and people will understand either way what you say if you use them interchangeably. i don't know if it's the same for korean
It serves as a noun, which 이, 그, and 저 cannot do on their own. If you wanted to just use 이 without the 것, for example, you would need a noun to go with the 이.
이 차는 비싸요 = This car is expensive.
Without a noun to go with the 이, you need the 것.
이것은 비싸요 = This is expensive / This thing is expensive.
You couldn't say something like 이 비싸요. You need the 것.