I think the audience for this is someone who is already using AWS Secrets Manager, but wants to reduce their API usage (perhaps due to cost).

Chamber uses SSM Parameter Store, which for many cases is similar, but some people might have a preference for Secrets Manager. For example, a team might like the automatic RDS password rotation for Secrets Manager and decide to put everything there for consistency.

For Doppler, well maybe someone doesn't want to pay for it, or they'd rather control access to their secrets via IAM instead of through a separate tool.