The User-ID feature lets you associate your own identifiers with individual users so you can connect their behavior across different sessions and on various devices and platforms. Analytics interprets each user ID as a separate user, which provides you with more accurate user counts and a more holistic story about a user’s relationship with your business.

References:

1. https://support.google.com/analytics/answer/9355972?hl=en&sjid=16127955654544478049-AP
2. https://support.google.com/analytics/answer/9213390?sjid=16127955654544478049-AP