postbac? I think you mean Master's programs?
Masters standalone does not boost undergrad GPA, which is (according to some people) more important than grad GPA. Post-bacc is taking undergrad courses, so it will raise undergrad gpa.
for some people with really low GPA like 2.xx and a lot of credit hours behind them, it is hard to do a post-bacc and pull it up without significant amounts of time/money so it is best to start anew with a Master's or SM program.