Facebook Pixel, Custom Audience and Server Tracking (GTM) Why are they necessary and what will happen if you don’t? Nowadays, online business means war, you either play with data. You need to understand – who is coming to your site, what is being viewed, who is buying and who is just leaving. Without this information, you are shooting in the dark. This is why it is very important to use Facebook Pixel, Custom Audience and Server-side Tracking (with Google Tag Manager).