So that drivers of cars driving in front of the ambulance see the correct inscription in the rearview mirror. In my opinion, a red cross and a loud crackling are enough.
It's a strange question, but it's as clear as daylight - resuscitators live in the looking glass, these cars have the right steering wheel like in England, where Lewis Carroll and his Alice are from.
In order for the drivers of other cars to see the inscription "Ambulance" in the mirror and know that the car must be passed. Another question is why in Russia such inscriptions are often written in English, this is really not logical. )