Bars, either horizontal or vertical, are intended to preserve the original aspect ratio chosen by the director. Tinkering with them is possible but it’s akin to deciding the Mona Lisa needs to be zoomed or cropped to fit your display rather than examining it as the creator intended. Either way is a distortion of the original.
I’m not suggesting that a TV show or movie is the artistic equivalent of the Mona Lisa, of course. But on a purely practical level, what I stated remains true.
There are rare exceptions, mostly where some widescreen compositions were hard coded within a fixed aspect ratio (early non-anamorphically enhanced DVDs suffered from this problem and were “windowboxed” within a 16:9 HDTV). Today, however, if bars have “disappeared” it’s either the broadcast that’s been cropped/zoomed, or the viewer altering settings on a display. Many people think the bars are robbing them of the complete image when the opposite is the case Even the rare “windowbox” appearance still provides a full image, albeit smaller than expected.