Since 2018, immigration cards are completed automatically for you at the border entry point.
The card contains your visit details and is in 2 identical parts. One part will be retained by the Immigration Officer. The other part should be kept safe with your passport.
There are many hotels and hostels that will not accept guests without an immigration card.
You will need to submit this at passport control when you leave Russia.
If you lose the card you can also be fined and your departure from Russia could be delayed.