The duties of a "valectus" ("yeoman") seem menial (like those performed by Arcite when he returns, disguised, to Theseus' court in The Knight's Tale), but the young groom who served well could, like Arcite (Knight's Tale 1438-40) or Chaucer himself, rise to the rank of royal squire. The fact that Chaucer was granted 20 pounds per year shows that his duties were not restricted to purely menial tasks.
There shall be eight valets of the chamber, that is, footmen, who shall make beds, hold and carry torches, and various other things which he and the chamberlain shall command them. These valets shall eat in the chamber before the king. Each of them, be he well or ill, shall have for his allowance one dole of bread, one gallon of beer, an ample meal from the kitchen, and yearly a robe in cloth or a mark in money; and for shoes, each shall have four shillings eight pence at two seasons in the year. If any of them be sent out of the court on the king's business, by his commandment, he shall have four pence a day for his expenses.
From Life-Records of Chaucer, Ch. Soc. II, 19 (As in Rickert, Chaucer's World). The original (French) text is published in T.F. Tout, The Place of Edward II in English History, 2nd. ed. rev. Hilda Johnstone. Manchester. 1926. p. 253 [Widener 1475 5.5].