The Torah commands in Parashat Emor (Vayikra 22:2) that the Kohanim must not “desecrate My sacred Name” by performing the Aboda (service in the Bet Ha’mikdash) in a state of Tum’a (impurity). This prohibition is especially strict, as violators are liable to Mita Bi’ydeh Shamayim (death by G-d, as opposed to execution by Bet Din). The Sefer Ha’hinuch explains that the Torah forbade Kohanim from serving in a state of impurity out of respect for the Bet Ha’mikdash, and for the Aboda. Allowing somebody impure to perform the service would compromise the honor of both the site and the service, and so the Torah forbade Kohanim from serving in a state of Tum’a. The Sefer Ha’hinuch lists a separate Biblical prohibition (Misva 363) forbidding entry into the area of the Bet Ha’mikdash in a state of impurity, and one who violates this command is liable to Karet. The question thus arises as to why an additional prohibition is necessary to forbid performing the Aboda in a state of Tum’a. After all, the Aboda, quite obviously, is performed only in the Bet Ha’mikdash, and thus once the Torah forbade entering the area in a state of Tum’a, nothing is seemingly added by a separate prohibition against performing the Aboda in a state of Tum’a. Tosafot in Masechet Shabuot (17) answer this question by envisioning a scenario whereby a Kohen is unaware of the prohibition against entering the Bet Ha’mikdash in a state of Tum’a, but is aware of the prohibition against serving in a state of Tum’a. If such a Kohen enters the Bet Ha’mikdash while impure, he has transgressed this prohibition inadvertently, and is thus not liable to Karet for entering the area in a state of impurity. If he then performs the Aboda, he transgresses the second prohibition intentionally, because he is aware of this prohibition. Therefore, although he is not liable to Karet for entering the Bet Ha’mikdash, he is liable to Mita Bi’ydeh Shamayim for performing the Aboda. Since the Torah issues two separate commands, forbidding entering and serving, the Kohen in this case would be liable to Mita Bi’ydeh Shamayim for serving in a state of Tum’a. This command, of course, applies only in the times of the Bet Ha’mikdash.