Sforno on Numbers 18:7/ספורנו על במדבר י״ח:ז׳

1תשמרו את כהונתכם. To ensure that a non-priest, such as King Uzziah, does not enter the domain reserved exclusively for the priests.

2ועבדתם, priestly functions are to be carried out only by you the priests;

3the very fact that this assignment is in the nature of a gift for you, עבודת מתנה, makes it likely that outsiders are jealous and want a share of this. This is why you have to be especially on guard (to prevent interlopers from being punished by death.)