Recently, Valve made changes to the way Steam works due to the British "Online Safety Act." From now on, access to games that are marked as "adult" projects will be closed to users from the UK unless they have passed age verification.
In order to confirm their age, players from the UK will need to link a valid credit card to their Steam account.
The system will consider the Steam account holder "verified" as long as a valid card is linked to it.
After passing the verification, users will have access to "adult content" that was hidden.
Valve explained why this method of verification was chosen:
In the UK, Ofcom is the independent regulator for online safety. Ofcom's guidance on the Open Source Act (OSA) states that one highly effective measure to ensure [verification] of age is credit card verification. This is because in the UK, a person must be at least 18 years old to obtain a credit card, so credit card issuers are required to verify the applicant's age before issuing them a credit card.
Keeping a credit card as a payment method provides additional protection against bypassing age verification by using one Steam account by multiple people.