Hello Katie. I am a independent midwife so can answer from my POV. I like to work with a doula. It gives the birthing mama a fresh person if the labour is long, so we can both have little rests so can therefore best support the woman. It is always (IMO) good to have a second pair of hands if the suitation arises that they are needed and this doesn't necessarily mean a emergency suitation. Someone else to help clean up afterwards.
You can still homebirth without either a midwife or doula. But if you want regular antenatal checks done such as blood pressure, abdo palps etc a midwife will do that, a doula cannot. If you want blood tests and ultrasound referral you have to go to a doctor a midwife cannot order those in Australia outside of a hosital system.