For example, if I live in the United States and someone asks me how much my shirt costs, it should be sufficient for me to say "Twenty" because it should be obvious that I am referring to dollars; however, I, personally, and everyone else I know, always says "Twenty dollars" rather than just "twenty". When asked how much one weighs it seems more common to just say a number, but it is not uncommon to say "150 pounds" rather than simply "150".
I am curious as to why it is so incredibly common (in fact, the standard) to say the unit of measurement even when it should be so obvious what it is.