It's still hard for me to fathom this valuation. For example, all the major clouds (AWS, GCP, Azure) have a Secrets Manager as simply one feature. I looked into 1Password secrets when they announced it but couldn't find any reason to use it over a cloud Secrets Manager.
For the same reason one might choose Hashicorp Vault versus the major cloud: cross-cloud, likely a richer feature set, almost certainly faster release cycles, and (for AWS specifically) no stupid "pay per request" billing to try and reason about. I'd guess it can make local development scenarios better, too