As a Tank, your gear traits are one of the few things you get to choose within organised group content. With organised groups you are usually expected to use specific things that they request you to use with regards to gear sets and skills. With gear traits this is one of your freedoms where you can utilise things for your own benefit.
Head, Chest, Legs and Shield all class as being “Big” slots of gear. When you apply a glyph to these slots, you get the full value of your glyph. These slots also typically give you more resistances than the “Small” slots of gear.
Shoulder, Waist, Hands and Feet are classed as “Small” slots of gear. When you apply a glyph to these slots, it will always be much smaller value than the same glyph applied to a “Big” slot.