Server-side Google Tag Manager moves part of your tagging from the visitor's browser to a server you control. That can give you real benefits. It also gives you a second container, a server, a domain setup and a monthly bill. It is not an upgrade that every site should take. This article explains what it changes, what it does not change, and how to decide whether your site meets the conditions where it pays off.
What changes when tags run on a server
In a standard setup, the browser loads each vendor's script and sends data straight to each vendor. With server-side tagging, the browser sends data to your own tagging server first. Google's introduction to server-side tagging describes a server container that runs in your Google Cloud project or another environment you choose. Components called clients receive incoming HTTP requests and turn them into events. Tags in the server container then send data on to vendors.
The browser part does not disappear. Google's course chapter What is server-side tagging? says you still need the Google tag running in the browser. The server only sees HTTP requests, so there are no click or scroll triggers on the server. You keep a web container and add a server container. The design of that data flow is a topic of its own and is not covered here.
What it can improve
Google's chapter Why and when to use server-side tagging names three goals: privacy controls, performance and data quality. In practice they mean the following.
