I'm guessing you haven't raided much. DBM will track, for instance, when you get into Naxx, when Anub is going to cast Locust Swarm. Wow doesn't tell you until he IS casting. Knowing ahead of time is helpful.
It will help the people be more aware when they're doing something obviously wrong, like standing in fire.
If you're not using DBM, you're half blind. If you know the fights really well and are already geared, no you don't need it to be successful. But you didn't need it in TBC either if you knew the fights.
If you have to ask this question though, yes, you need DBM.
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Karthal - Moon Guard - Warrior [A]
Katran - Moon Guard - Warlock [A]
Taurun - Moon Guard - Shaman [H]
Stygius - Moon Guard - Death Knight [A]
Katekat - Moon Guard - Paladin [A]
Karthal(FFXI) - Seraph Server - Warrior 75, BeastMaster 70