The voting registration card tells you which elections you can vote in and were your polling station is.

The voter registration card I think is also used (and needed) if you vote in advance, which you can do for the two weeks preceding the election at postal offices, libraries and such. You then also need an ID card, passport or driver's licence (ID cards are btw issued by banks in Sweden but are as valid as government-issued passports or driver's licence).

Maybe it is to make sure your vote is forwarded to the right polling station. If you both vote in advance and on the election day I think the later vote takes precedence.

If you lack a valid ID you can vote on the election day if you bring someone can ID themselves and who testifies that you are who you say you are.

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