It's mainly mitigating exposure. Some possible vulnerabilities would be social engineering(i.e. it'd be easier to send a targeted phishing URL to gain recon on

It's mainly mitigating exposure. Some possible vulnerabilities would be social engineering(i.e. it'd be easier to send a targeted phishing URL to gain recon on an employee of a company if you know an internal domain), or injection into a public facing service that has access to internal services.