If One Does Not Learn To Apply His Knowledge
Rabbi Micha Golshevsky commenting on "A Greater Obligation":
Chazal teach that study is of great value since it leads to action. Clearly, one should learn not only to fulfill the mitzvah of Torah study but also with a view to changing his actions. It is for this reason that many authorities state that the first thing one must work on mastering are the halachos of Orach Chaim. Without these halachos one could be the greatest lamdan but have no idea how to really apply his learning.
The Chayei Adam, z"l, even writes that it is better to learn the halachos of Shabbos on Shabbos than Mishnayos. To illustrate why, he recounts a revealing story. It is first important to realize that although he served as the Av Beis Din of Vilna, the Chayei Adam was a businessman who never took any money for deciding halachic queries, just as his father before him. As a businessman, he traveled frequently. One Shabbos, he stayed in the same inn as a person whose practice for many years was to learn a chapter of Mishnayos every day.
Understandably, the Chayei Adam was appalled when he noticed this "expert" in Mishnayos weaving on Shabbos! He immediately cried, "Is it not Shabbos today?"
The man was puzzled. "But what possible melachah can this be?"
"How can you be so unaware? Are you not familiar with the mishnah which lists 'hatoveh' as one of the melachos?"
"But I thought that was only if someone does so on a loom like we do at home…"
The Chayei Adam was astounded. "But having learned the mishnah, why would you assume that seeing that it simply says 'he who weaves' implies that weaving is only a melachah with a loom?"
"Do you think when I learn I am trying to apply my learning to my actions? I only focus on fulfilling the mitzvah of learning Torah," the man protested.
The Chayei Adam responded, "Now I understand the words of our sages: 'One who says I only have Torah does not even have Torah.' If one does not learn to apply his knowledge, what earthly difference is there whether he learned or not?"
Much like halacha, mussar is the practical application of Torah into action since it is impossible in our day to have a properly balanced relationship with Hashem or one's fellow man without a genuine path in mussar or Chassidus.