Do clarify the rules, as they can (and often do) set limits on age, “suitability” and even maximum CO2 levels.
As said, allowance is taxed as income so may not be as generous as you might think. Unlike what b r says, they can reimburse at whatever rate they like – usual thinking is that car allowance is to cover car payment & insurance, etc so mileage payment can then be fuel and something towards maintenance. You can get a bit of tax back if your company pays less than HMRC rates though.
Benefits – you can run what you like within reason, no silly restrictions on fitting towbars, etc. If you’re not fussed about being in a new 3-series every few years then you can often run something for less than you’re being paid and spend the excess on bikes.
Pitfalls – you insure it (and suffer the increased premiums if someone breaks into it while it’s in a hotel carpark somewhere), you maintain it, should you be sacked or made redundant the car is still yours and you keep paying for it.