Is there a doctor in the house?
Well, there's Cornelius, but he's not necessarily going to give you the medicine you ask for. When the Queen asks him for a potion that can kill, he serves one up... except that it's a more of a sleeping potion than a death potion—whoever takes it will wake up after a little while.
Cornelius is a clever dude: he knows he'll get in trouble (translation: he'll die) if he crosses the Queen, so he outsmarts her instead. He figures she won't know the potion isn't what she asked for until she gives it to someone—and then he'll have time to confirm his suspicions about her. The thing is, Cornelius also gives us an idea of what the general public thinks about the Queen. So what if Imogen hates her? Isn't that required of stepdaughters, anyhow?
Cornelius shows that the Queen truly is an awful person, and the general public knows it. He's really just there to help us learn about other characters and to fill in the narrative for us at the end. Only he knows about the secret potion he made for the Queen (and that she gave to Pisanio, who gave it to Imogen). Without him, poor Pisanio would probably be blamed for the Queen's plot.
Maybe he's mostly a plot device, but this doctor gives us the news, all right.