Fixed headers are banners or navigation bars that stick to the top of the screen instead of disappearing when you scroll down the page. They’ve become popular since the web moved onto mobile phones.
I think they’re normally used for one of two reasons. On a long page, or on a screen that makes for lots of scrolling such as a phone, you could argue they make navigation easier. They’re always available, whatever your position. Secondly, they’re quite whizzy. We expect pages to scroll beyond the browser window and exist in some abstract space, to be called back into view with a swipe or flick of the mouse wheel. Fixed headers stay concrete; they’re always present on the screen, which perhaps feels interesting.
In my opinion (no science here, I’m afraid) they’re of limited use. Finding the top of a page is normally pretty easy regardless of how much you need to scroll; in return for the marginal convenience of fixed navigation, you give up space for content. (We might note how adaptable we are when it comes to new interfaces: an abstract notion of a page existing beyond a visible space is actually pretty complex.)
A short fixed header probably makes little difference if you’re reading prose on a screen. But if you’re looking at an image you need as much page as possible the taller the image gets, and you want to avoid scrolling. This can easily become a problem on a mobile screen, but the Uniqlo website header is so tall it even affects my Macbook Pro:
Viewing category pages and close ups of clothes on the Uniqlo site is difficult because the fixed header restricts the available space. This means you can't see whole items.
Lots of clothes websites use fixed headers, which is strange considering they need as much screen space as possible. Uniqlo’s is the worst I’ve come across yet, making the site largely unusable.