Originally from T-Line Guide
The purpose of T-fitting serves a few main reasons:
1. It is designed so that you can fill your system initially with water. You pour water down the tube, to prime the pump, and the pump circulates the water throughout.
(Red=where to pour water and where it would go. Blue is where it circulates.)
2. It is consistently used there after to bleed the system. Bleeding is basically removing the air bubbles from the water, increasing the heat conductivity.
(Blue= Water flow. Red=Location of where bubbles go.)