Jamieson, Fausset and Brown on Deuteronomy 18:6-8

6-8. if a Levite ... come with all the desire of his mind--It appears that the Levites served in rotation from the earliest times; but, from their great numbers, it was only at infrequent intervals they could be called into actual service. Should any Levite, however, under the influence of eminent piety, resolve to devote himself wholly and continually to the sacred duties of the sanctuary, he was allowed to realize his ardent wishes; and as he was admitted to a share of the work, so also to a share of the remuneration. Though he might have private property, that was to form no ground for withholding or even diminishing his claim to maintenance like the other ministering priests. The reason or principle of the enactment is obvious (1Co 9:13). At the same time, while every facility was afforded for the admission of such a zealous and self-denying officer, this admission was to be in an orderly manner: he was to minister "as all his brethren"--that is, a Gershonite with Gershonites; a Merarite with Merarites; so that there might be no derangement of the established courses.