What's this Sherlock "commentary" business all about then?

[Hotbots]

I get that the episodes are one and a half hours long, but what are the commentaries and what are they for? Are they just audios, but of what, what is there to say for an hour and a half?

How odd, why has my quote sign gone like that I wonder?

If they're like the ones on the Doctor Who DVDs, it's various combinations of actors, producers, directors and 'back room people' talking about how things were done, stories about what happened on set - depends on who's doing it. Actually I'm watching a commentary on The Big Bang (last episode of series 5 of Doctor Who) right now - it has the director of the episode with Kearen Gillan and Arthur Darvill, so there's a lot about camera angles and takes. This one is an in-vision commentary, so you can see the people in an onscreen box, but most of the ones I've seen before have been audio only.