Imagine instead of pressing backspace to delete a character, you would have to delete the individual bits that make up the character. A character is just an abstraction of bits, that your text editor/IDE takes care of for you so you don't have to think about it.
Vim is also an abstraction, making it similarly far more convenient than doing everything by hand. Think about it: Is "forward 2, backspace 5" or "delete this word" a more natural abstraction?
So what's the advantage? Speed, but not typing speed. It's about the mental distance between the model in your head of what you want to happen (delete everything between the parens) and the model your tool understands (go forward some words, go forward 1, delete backwards 20). Since speed = distance per time, reducing the mental distance increases mental speed.
And that therefore uses less mental energy on translating for your tool, and leaves more room for doing hard mental problems, like fixing a bug.