Once upon a time they used to release movies in cinemas.
Then they started to rerelease them on Super 8 and VHS.
Then they started to rererelease them on VHS, but with additional sequences.
Then they started to rerererelease them on DVD, but with even more additional scenes.
Somewhere along the line, probably on the quiet, new movies began to have additional scenes shot specifically to be embargoed for this purpose. I cannot prove it, but I have grave suspicions about the single-take extra scene included on the Batman & Robin DVD that I borrowed from Herschel. Conversely, I understand that the faux extras on The Hitchhiker's Guide To The Galaxy had no shame at all about themselves.
Which brings us to Doctor Who: Meanwhile In The TARDIS - two self-contained extra scenes specifically shot to be DVD extras. They're both written by the series' head writer, both filmed on the TARDIS set, both featuring just the Doctor and Amy, and both bridging the gap between two specific episodes. They're minisodes.
And they're also both very talky, which negates their suitability for the frantic pace of the actual show these days.
After taking off at the end of The Eleventh Hour, Amy grills the Doctor on all those questions that new viewers might be asking. Why does the TARDIS look like a Police Box? What can you see through its windows? It's made of wood, so why doesn't the air inside escape? Finally the Doctor shoves her out of the doors into space, leading straight into The Beast Below, in which there disappointingly turns out to be a forcefield.
After taking off at the end of Flesh And Stone, Amy grills the Doctor on all those questions that new viewers might be asking. Why he won't go to bed with her? (uh, because you cheat on your men Amy?) This leads into the deeper and far more interesting question of why the Doctor travels with a companion, and he's forced to admit that he might not be quite so aloof as he likes to think.
Doctor: "I look at a star and it's just a big ball of burning gas, and I know how it began, and I know how it ends, and I was probably there both times. (Y'know) after a while everything is just stuff. That's the problem - you make all of space and time your back yard, and what do you have? A back yard. But you - you can see it. And when you see it, I see it."
Amy: "And that's the only reason you took me with you?"
Doctor: "There are worse reasons."
Amy: (splutters) "I was certainly hoping so."
Amy accesses the TARDIS' visual records of its previous occupants, only for it to cheekily show her just the girls. This catalogue even includes Liz Shaw, who arguably never travelled in the TARDIS.
Finally the Doctor sets the TARDIS' controls for Rory's stag night, leading straight into The Vampires Of Venice, which disappointingly turns out to be miserable. In fact, the inclusion of this scene on the beginning would have significantly improved that story, by providing evidence of the Doctor's pure motives, and therefore a much nicer context for events.
Still didn't explain why he hid in the cake though.