It lets organizations (Tailscale) control the timing and narrative around the disclosure more directly. Organizations sometimes avoid the bureaucracy of going through CVE Numbering Authorities by self-publishing. Often a CVE assignment follows self-disclosure, especially when there's pressure to interoperate with vuln-scanning/compliance tooling
In my experience it is really very quick and easy to get a CVE if you contact MITRE directly, and on paper you do not need to disclose too much. The GitHub CNA is (ironically) very slow by comparison but that is a GitHub issue.
Tailscale can also just apply to be their own CNA and issue CVEs for their products themselves, eliminating any such issues entirely.