It's a formal subject, not referring to anything in particular. It can be compared to the it in it's raining. It just has to be there to make the sentence complete.
Just been reading your excellent post linked above. As I understand it, I therefore could say "they are my bags" "de är mina väskor", if I'm emphasising 'my/mina', and referring to bags that have already been mentioned in a conversation (e.g. "Are they Bob's bags?" "No, they are my bags")
If, however I'm introducing my bags to the conversation for the first time, then the 'det är' construction at the start of the sentence is independent of the subject (the bags), and I should use "det är mina väskor".
This kinda makes sense to me now (I hope), but I don't think I'll ever get the hang of saying 'it is my bags' in the heat of the airport queue.
The conversation about those bags would still be Är det Bobs väskor? Nej det är mina väskor, unfortunately. Saying de there sounds unnatural. But if you skip väskor in the second sentence you get to say de: Är det Bobs väskor? Nej de är mina. Then you've changed it from a new topic to something known.
I know it's counter intuitive, I can't help thinking the English usage here is really weird.
Wow, ok! Well - no-one ever made a grammar rule because it was logical, but because 10 million people use it. Thanks for your input! have a lingot...
actually I meant the swedish 10 million... I often find myself arguing with a swede about how the grammar doesn't make sense, but then I remind myself that a) they didn't create it, b) language evolves, it doesn't get built, and c) english makes less sense.
English is spoken by a lot more than 10 million :D (seriously, I see what you mean, but I think it's English that is being illogical).
Exactly. those is 'de där' (in this case, det där) and these is 'de här' (in this case det här).