Per Deuteronomy 18:6, a Levite may choose his own time of service at the Temple. He can't be drafted or forced to serve. Once there, though, he doesn't necessarily get to choose his own job. This is a lot like surrendering to God. He calls, but he won't force you to answer except under extraordinary circumstances. But once you have committed to serve him, you don't get to dictate the terms of your service. It's all or nothing.

Vs 6-8 imply that a Levite is only entitled to a share in the firstfruits offerings when he is actively engaged in service at the altar. This also suggests that we should materially support those engaged in spiritual ministry in God's Kingdom.