Hester hopes to save Dimmesdale from continued suffering at the hand of Chillingworth. In Chapter 14, we see Hester and Chillingworth speak of Dimmesdale. Hester knows that Chillingworth has figured out that Dimmesdale is Pearl's father, and Chillingworth has been getting revenge on Dimmesdale by making him suffer psychologically. This psychological suffering (Chillingworth and Dimmesdale spend much time talking of secret sin and guilt) is making Dimmesdale suffer physically as well. When Chillingworth refuses to leave Dimmesdale allow, Hester decides to tell Dimmesdale that Chillingworth is her husband. She hopes that by having this knowledge, Dimmesdale will be able to stand stronger against Chillingworth's attacks.
Hester hopes to save Dimmesdale from furthering suffering at Chillingworth's revengeful hands. For the first time, Hester admits her true feelings about her husband and sees what his seven years of vengeance have done to him. She wants Dimmesdale to finally know the truth.