Be careful with the 'utility' model of explaining behavior. It is fairly easy to slide into 'if behavior X is manifested, this must mean X must somehow be useful'. You can use this model to explain behavior, but be aware of the circularity trap in the model. "She lied thus the lie must have had use, even if it is not obvious we will discover the utility if we dig down enough".

Another model can be post-rationalization. People just do stuff instinctively, then rationalize why they did them after the fact. "She lied without thinking about it, then constructed a reasoning why the lie was rational to begin with".

At the extremes, some people will never lie, even to their detriment. Usually they seem to attribute this to virtue. Others will always lie. They seem to feel not lying is surrendering control. Most people are somewhere in between.