IAM control plane for example:

> There is one IAM control plane for all commercial AWS Regions, which is located in the US East (N. Virginia) Region. The IAM system then propagates configuration changes to the IAM data planes in every enabled AWS Region. The IAM data plane is essentially a read-only replica of the IAM control plane configuration data.

and I believe some global services (like certificate manager, etc.) also depend on the us-east-1 region

https://docs.aws.amazon.com/IAM/latest/UserGuide/disaster-re...

In addition to those listed in sibling comments, new services often roll out in us-east-1 before being made available in other regions.

I recently ran into an issue where some Bedrock functionality was available in us-east-1 but not one of the other US regions.

IAM, Cloudfront, Route53, ACM, Billing...