G‑d told Moses to conduct a test that would demonstrate the error of Korach and his supporters. Offering up incense was part of the Tabernacle service that was allowed to be performed only by the priests, and only at specified times. The Jewish people had previously witnessed how Aaron’s two elder sons had died on account of having offered up an unauthorized incense offering. Now, G‑d had Moses instruct Korach and his company to offer up incense, as would Aaron, also. Whoever’s incense was not accepted by G‑d would perish.
Holy Rebels
וּקְחוּ אִישׁ מַחְתָּתוֹ וּנְתַתֶּם עֲלֵיהֶם קְטֹרֶת וְהִקְרַבְתֶּם לִפְנֵי ה' אִישׁ מַחְתָּתוֹ וגו': (במדבר טז:יז)
Let each man take his censer and place incense upon it, and let each man present his censer before G‑d. Numbers 16:17

Although they knew that unauthorized use of the incense would cause their death, Korach and his followers took up the challenge. They wished to experience the lofty service of the high priest even if it would cost them their lives.

In this sense, their motivations were pure, and we can learn from their example, similarly aspiring to the most uplifting spiritual experiences. Their error, which we must also learn from, was disregarding the absurdity of going against G‑d’s will in order to get close to Him.1