It was a great move on her part, the opportunity cost was pretty low (taking a smaller contract for 1 extra year) and the reward was strong (bolstering her resume with some NCAA success and visibility). She signed a huge pro contract a year later.
NCAA sports are probably more popular with high schoolers and college students than pro track is, so that boosted her visibility with that group... who probably buy a decent amount of NB merch. Alumni also have a strong connection to their school's athletic program, so when a star athlete like McLaughlin comes to their school (even in a smaller sport/event) it definitely creates some publicity for her.
She also probably diminished expectations a bit by following the natural progression of most athletes, even if she only went to college for a year. Look at how much scrutiny Drew Hunter is under because he decided to go pro and skip college completely. Deviating from the "set path" brings much higher expectations.
Financially, I don't think she paid anything at all to go there, as she was THE top recruit that year, and I think she/her family understood the benefits of college athletics vs going pro right away.
Finally, there were definitely personal reasons, such as meeting people, experiencing college life, running with a team ... that helped her grow as a person/athlete.