What does Odysseus promise to the drover and the swineherd in the Odyssey, and why does he make the promise?

In book
21, Penelope decides to set up an archery contest between the suitors to choose who she will
marry. Penelope retrieves Odysseus's bow and sets up twelve ax heads in a row. Penelope promises
to marry the suitor who can successfully shoot an arrow directly through the ax heads using her
husband's bow. Several suitors attempt the feat but cannot even draw Odysseus's bow. While the
suitors are struggling with the bow, Odysseus bides his time and remains disguised as a lowly
beggar.

Odysseus then summons the cattle herder Philoetius and the swineherd
Eumaeus into the courtyard, where he tests their loyalty before revealing his identity. Odysseus
tells the men that he has returned home after twenty years and will finally cleanse his palace
of the suitors. He then promises Philoetius and Eumaeus that he will give them each a wife,
cattle, and build houses for them near his palace. He also promises that they will become
kinsmen of Telemachus before showing them his scar as proof of his identity.

Odysseus makes this promise in order to cement their trust and gain their assistance to
defeat the suitors. Odysseus is also recognizing and rewarding their loyalty. After being gone
for twenty years, Philoetius and Eumaeus have remained loyal to him and have done their best to
protect his household. These gifts serve as social and economical improvements for both
servants, who proceed to help Odysseus reclaim his home.