Laserfiche Single Sign (requires the STS) mainly brings the following benefits:

• If users access multiple Laserfiche applications, they only have to log in or out once
• Ability to use more advanced authentication options, such as multi-factor authentication and SAML and AD FS authentication
• Support for subscription users that have lower functionality licenses in Forms and the repository. These users cannot log into Forms without SSO

For example, you mentioned that your external server has users type in their (Windows?) username and password. A better security practice is to set up AD FS, which is a Windows feature designed to securely authenticate Windows users that are outside your internal network. This would only available through the STS.

However, for the use case you described, it's not strictly necessary. We do have further updates planned to reduce the difficulties with setup.