Tritanium is a much stronger and efficient armor in comparison to Plasteel Plates. They are commonly used by low level players replacing Plasteel plates, equipping them on almost all modules. Lighter variants are more efficient mass to armor ratio than in comparison to Plasteel.
However Tritanium Plates is almost instantly outclassed by Iridium Plates which offers more armor and a rather moderate repair. Tritanium is usually unused in mid-level bases because it doesn't give enough armor.