what is this "Authentication Request" on IMDb to make post ?

I get a bit confused. I must give up my credit card number or my phone number to make posts. IMDb says its free, but further down it says …

I have 2 choices: either to give my credit card number om my mobile phone number ?? - I will certainly not give them my credit card number

could anyone explain this ?

I used to be a frequent poster on the IMDb messageboards. I believe the “authentication” of new posters is optional on the boards, but mandatory for those who want to edit or add to the FAQs and other non-messageboard areas of the site.

IMDb used to have a huge problem with trolls and spammers, and the “authentication” process has improved things dramatically by making it more difficult for people to sign up under multiple usernames for the purpose of creating mischief. I gave my credit card number to IMDb, and nothing bad has happened. I haven’t heard of any incidents of misuse. The only way anything will be charged to your card will be if you choose to join IMDb Pro (a subscription service that offers in-depth info, mostly useful to those in the entertainment industry).