All of your account information is contained on your Roku account, on Roku's web server. A browser must be used to access websites, and there is no browser for the Roku system. That's why you need to create an account and log in via a mobile device which does have a web browser. When/if you add additional Roku units, they are added to this same account without having to re-enter all your info.
CC info is requested to cover incidental expenses you may accrue, such as channel subscriptions you may enter thru the Roku (as opposed to directly with the service provider), movie rentals, etc. This is like checking into a hotel where you have a prepaid reservation -- they still want a credit card in case you charge something. After your Roku account is created, you can always log into your account via https://my.roku.com/payment/account, scroll down and click "Remove payment method".
It is not well publicized, but you can create a Roku account without a credit card in the first place via https://my.roku.com/signup/nocc
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