Why do they have a car allowance in packages?

why do most manager + grades in big 4 have a car allowance added as cash? why not just increase the basic?

Is it just to give a lower basic and therefore subject less salary to % increase/bonus?

car allowance is normally not pensionable or included in bonus etc if given as a %, it's often non guaranteed unless if written into your terms of employement, it's usually cheaper for them than giving you a company car

Usually a car allowance carries with it the obligation to provide a car of the appropriate standard.

For example my last employer specified that if you took the car allowance that you had to provide a 4 door car less than 3 years old with insurance for business use. If it was off the road you had to hire a car at your own expense.