My take:
Its purpose is there for employees who will be using their own vehicle for business purposes, to get the allowed tax break for such business travel. Generally, it wont be added as a "on top of your salary" but rather as part of your salary. So it isn't anything extra but part of the package. If you actually have to travel a lot, your travel allowance would increase your salary, but likely only what your employer would think the travel expense would be. For example, person A and person B both work for the same company, doing exactly the same job. Both have to travel for work. The company has a 3 series BMW fleet car, which person A uses after he gets to work in his own car. Person A gets a R20k salary. Person B has to use his own car, so lets say the petrol he drives out that he has to pay himself, amounts to R1500 per month on average and lets say a generous wear and tear and maintenance of R500 to R1000. His salary might at most be R23k which is made up of R20k salary and R3k travel allowance. They might make it more attractive and give a R5k allowance and R18k salary. Point is its still R23k and only R3k more than the other guy and not enough to buy a new BMW.
If you use your own car for business, you are allowed a tax deduction for your business travel expenses, and only if you receive a car/travel allowance. The deduction is also limited to the travel allowance and effectively the reduction in tax is limited to the tax you paid on the travel allowance.
The correct term is travel allowance, rather than car allowance because its purpose and the way the deduction works is to cover your travelling costs such as petrol and maintenance and a little wear and tear. It is not meant to reimburse you the cost of a new car as well, as the latter would suggest. Companies often lure employees with say, a R8000 monthly travel allowance, and the employee would think he can now buy a R8000 monthly installment car. If you dont regularly travel actual business kms then the allowance is the same as normal salary and you probably will have to pay in a lot of tax at the end of the year.