I'm not necessarily a fan of Apple's design, but I want to add that when you have floating header bars it cuts down screen real estate and makes the UI feel more claustrophobic. Making it semi-transparent helps that significantly.
There are usability reasons for this too - for instance, even if it's blurred, a hint of what content is behind the bar helps the user know when they've neared some new content or when to stop scrolling, or whether there's more content above/below the unobscured viewport.
There are usability reasons for this too - for instance, even if it's blurred, a hint of what content is behind the bar helps the user know when they've neared some new content or when to stop scrolling, or whether there's more content above/below the unobscured viewport.