The main use as far as databases go is to ensure uniqueness of keys across databases. This is important if you have multiple servers or disperate systems that need to replicate or share
data and each can generate data on its own end. You want some non-centralized mechanism to ensure the ids generated from each server will never overlap.
There are various open standards
for generating these ids and each standard will tie the id based on some unique identifier of the computer or a namespace or just a purely random generator algorithm not tied to anything.
Since this is a question often asked by users coming from Microsoft SQL Server, we will demonstrate in this article the same concept in Microsoft SQL Server and how you would achieve similar functionality in PostgreSQL.