I am not talking in terms of what baali teshva are looking for "spiritual trips." Rather I am referring to the basic need to explain what the Torah asks from us as simple plain Jews that need its set of values and hierarchy of values explained. Without these three people one could take the Torah to mean anything at all, and in fact that is exactly what baali teshuva do.
What each did was reveal an important aspect of Torah, The Gra: learning Torah. Reb Salanter: Midot good character traits. Rav Shach: How to learn.
In plain English what I am saying is that you learn from these three people three important things about Torah: (1) It is very important to learn Torah and avoid Bitul Torah. (2) To be a "mench" good character traits. (3) To learn Rav Shach's, Avi Ezri shows how to learn Torah.