From the Party's point of view, "ignorance is strength" because it is a mode of survival. The ignorance of Oceania citizens enables them to survive. If they emerge into the realm of knowing the truth and seeking to activate their voice, Winston's narrative demonstrates how bad things will get for them. It is in this light where the Party demands of its citizens to show "strength" in remaining creatures of "ignorance." The idea behind the strength of ignorance is an example of Doublespeak, language meant to confuse the body politic into a state of submission.
The advantages of being ignorant boil down to survival. Given the lack of fear in terms of using brutality and torture as a means to secure individual submission, being ignorant is something that avoids this condition. Consider the Prole woman who is putting up laundry moments before Winston and Julia are found. She would be seen as ignorant. She is able to sing freely, go about her life in a lack of consciousness and she is left alone. Certainly, the fate that Winston and Julia suffer as individuals who "know" is far more brutal than hers. The need to survive becomes one of the dominant advantages in being ignorant. This is the state of being that Big Brother has constructed for the citizens of Oceania. Certainly, Orwell is not suggesting that it is good to be ignorant. Yet, in the condition of Oceania, if one wishes to live and survive, being ignorant is the only path one can pursue.
There might be another metaphysical advantage to being ignorant. In being ignorant, one lives in a world of oblivion. They are not aware of the horrors that exist in the world. Consider the strength in one's being that emerges when one is ignorant. Failure to understand the implications of Room 101, avoidance of the Thought Police, and being able to not have to undergo the trials that Winston and Julia endure all represent distinct advantages of being ignorant. Being ignorant has the advantage of being unaware. Again, this is not something that Orwell embraces. Yet, he is suggesting that the condition of Oceania is one in which individual liberty has been removed. A state of being like this one is one in which being ignorant has its advantages for any semblance of seeking to be human or being knowledgeable carries risks of brutality and pain.
For his part, Winston is shown to possess the human trait of seeking to remove his own ignorance. The exposition of the novel is one in which Winston shows himself to be one that rejects the idea that ignorance is strength. For Winston, being able to remain knowledgeable about Big Brother and have a hand in bringing it down is where strength lies.