It’s a mechanism that is compulsory in EU (and nearby countries like Switzerland).
When you try to spend money online (without the plastic card), you need a second factor. In practice in, let’s say, Amazon there is an iframe with a page of your bank that asks to confirm the operation on the banking app or insert the code they sent you by SMS or things like that
Required: only in EU Available: EU close countries (UK, Switzerland, etc)
In the US banking is a bit different than over here. People still pay rent with checks (that in EU are de facto obsolete) possibly sent in an envelope via mail.
You may wonder why…. Because a money transfer (that in EU is generally for free) in US is often a double digit operation.