Why is the Talmud so important?
Rabbinic Judaism believes that God revealed to Moses two Torahs: the Written Torah (Torah shebikhsav, which is contained in the Five Books of Moses, also known as the Pentateuch or the Torah) and the Oral Torah (Torah shebe-al peh). The Written Torah cannot be understood without the Oral Torah. For example: when God commanded that the people put mezuzos on their doorposts, what does it mean? How is it done? The Oral Torah explains all of this.
Until Rabbi Judah the Prince (Rabbi Yehudah Hanasi), most of the Oral Torah was oral. But as the Jews began to be dispersed, and there was the fear that the Oral Torah, or portions thereof, might be lost, the sages decided to write it down. This resulted in the Mishnah and, ultimately, the Talmud.
Because the Written Torah is considered to be unintelligible without the Oral Torah, Rabbinic Judaism places great emphasis on studying the Oral Torah. Indeed, many Orthodox Jews are more familiar with what the Oral Torah says than what the written Torah says.
The Oral Torah is the lifeblood of Judaism. Without it, they wouldn’t know how to practice what the Written Torah commands them, and without the perspectives and insight provided by the Oral Torah, Judaism would not have been able to reform.
Indeed, the Oral Torah is considered to be as authoritative (if not more) than the Hebrew Bible. This is why Jews spend so much time and effort to study the Torah.