I don't get why these bars are being used everywhere
I Blame it on the design patterns of (Twitter) Bootstrap. A lot of sites are put together with it. Although the default nav-bar is not bolted to the top it is easy to read how to do that in the docs and implement it because it seems like a good idea at the time. Others, not using Bootstrap then copy the 'new convention'.
Personally I believe that for some content, e.g. a Tumblr style blog, the nav-bar at the top (and fixed) makes a lot of sense. If you also have pet-peeve infinite scroll (as many do on Tumblr blogs + Pinterest), then it is helpful to have a fixed header as there is no footer.
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I Blame it on the design patterns of (Twitter) Bootstrap. A lot of sites are put together with it. Although the default nav-bar is not bolted to the top it is easy to read how to do that in the docs and implement it because it seems like a good idea at the time. Others, not using Bootstrap then copy the 'new convention'.
Personally I believe that for some content, e.g. a Tumblr style blog, the nav-bar at the top (and fixed) makes a lot of sense. If you also have pet-peeve infinite scroll (as many do on Tumblr blogs + Pinterest), then it is helpful to have a fixed header as there is no footer.