Because no one like to be a party-pooper, Iago sings drinking songs in order to encourage Cassio to get drunk.
Drinking songs tend to be rollicking, jolly justifications of drinking.
Iago's second drinking song is a little more subtle. It's an old ballad which starts by making fun of a king who got angry because his cheap breeches cost sixpence too much. But being of high degree isn't necessarily a good thing, because it's pride that ruins a country. The conclusion is that you should warm yourself in your old cloak, and drink. After all, no one's too good to take a drink.