Because when you see it in your rear-view mirror, It reads AMBULANCE, and you know to pull over to let it pass.
It is spelled backwards on the front so that cars in front of it can read the word the right way around using their rear view mirrors :)
'Was' is 'saw' spelled backwards.
Gazelle spelled backwards is ellezag.
'Foxes' spelled backwards is 'sexof'.
'Quick' spelled backwards is 'kciuq'.
'Maps' spelled backwards is 'spam'.
Suez spelled backwards is Zeus.
Trey spelled backwards is YERT!
Kori spelled backwards is irok.
Taco cat spelled backwards is tacocat. And racecar spelled backwards is racecar. Cool right!
The word ambulance on an ambulance is backwards because when you're in front of an ambulance and there's an emergency you would usually see from your rear view mirror. When you see something in a mirror it is always backwards. so when you see the word ambulance on an ambulance, you would see it normally.
Stressed spelled backwards is desserts. Yum.