Yes, you are right; they are working together for the same goal, and here comes the question, HOW? If that means that I'm a DP and he's a healer (for example), we work in different ways; he should aim to get items to do his job better (heal items), and I should aim for items I need to do my role better (dps).
As simple as that, our goal is to kill the boss and progress further into the raid. His role is to heal people and keep them safe, so obviously he needs better items to do it better and easier, and he 100% deserves that, the same way I (dps) deserve to roll items that allow me to do my job better. Why am I losing the possibility to progress and grow because he wants items for other puproses? In guild, that can actually make sense; in common raid from global, its just unfair; you can't argue with that.