Because it may lead people to believe that all image processing will be done at full range, which it does not depending on some filters and transitions.
It does not unless your clips have incorrect color range signaling. With proper signalling, the pixel values will be requantized/scaled to the current range within the processing pipeline with no clipping. If the clip’s color range is incorrectly signaled or misdetected then clipping will occur. It sounds like this is more your issue than the output range. Look at your clip’s Properties > Video > Color Range. Please be aware that the current version 19.01.27 has a bug such that when you change this color range to override it, Properties does not correctly reflect your change! This is fixed in the next version, for which there is currently a beta.