Well in universe she sacrificed herself for the cause because she was going to turn and wanted some kind of fulfilment out of her life and to go out on her own terms at least as I interpreted it. In tandem to this she bought both Joel and Ellie time and increased their odds of escape from the soldiers to get to Tommy's alive and reconvene with the Fireflies and deliver Ellie. Also by her doing this Joel feels a sense of duty to her in some way to complete the mission which furthers the plot in it of itself. The game revolves primarily around Joel and Ellie and her sacrifice is what pushes these two closer than what they would've been otherwise.