Very very rarely now, but I'm hanging on to a few boxsets I bought years ago for stuff that I still like to watch regularly. Sometimes these boxsets have a decent package of extras; and sometimes, although the episodes may get repeated occasionally on terrestrial TV, they are often edited for content or just to include more ad breaks etc, so the transmitted versions aren't complete. Also some things may never be seen again in their original format - either because tastes change (perceived racism, homophobia etc), or issues with some of the actors - will we ever see a John Barrowman or Noel Clarke 'Doctor Who' shown again?
I am collecting a few of the classic 'Doctor Whos' from the 1970s on Blu-ray because they've made some advances in improving the picture quality or added some comprehensive documentaries etc, but these are pretty much the exception.