If I pinky promise to indent my code anyway, then why does it matter whether I also have braces or not? In fact, braces allow me to press one button in my editor and get the indentation absolutely perfect, without affecting semantics.
Braces also allow for easily copying and pasting blocks of code because the braces delimit the semantics of the copied text. Because your code is already indented, with white space indentation you have to check that a) you pasted the first line at the right indentation level b) every subsequent line is also at the right level relative to the first line. No small feat.