'Rashi': They were appointed over the chambers, and also the chamber of lambs where Korbanos are kept. Each day they brought to the Kohanim the number needed each day for the Tamid on weekdays, and also Musafim on days with Musaf. They also help to flay the Korban - "ha'Kohanim Hayu li'Me'at v'Lo Yachlu Lehafshit? va'Ychazkum Acheihem ha'Leviyim?"

Radak: They sang whenever they offered Olos [of the Tzibur]. When there were many Korbanos, they helped to bring Korbanos, do Shechitah, and all acts until Kabalas ha'Dam. From Kabalah and onwards is Avodas ha'Kohanim.

Malbim: They checked the lambs [for blemishes], and supervised flaying and dissection.