Depends on the car.

The purpose of rear spoilers is for downforce. Most cars do not need them
and they are just for show - to make the car look faster.
I think the Mark 4 Toyota Supra was the first to have the offensive sized
one, which has also been used on some Alfa 156's, and lots of other cars as
an aftermarket addition. (I remember reading once, years ago, of how such a
rear wing was stupid on any car but a Supra!)

There is a very good reason for the supra wing being so large though - it
doesn't obstruct the rear view, as it goes around the window instead of
through the middle of it.

At high speed, even a small spoiler can make a massive different to a cars
safety and handing. The Audi TT was recalled after multiple high-speed
accidents on the German Autobahns. The recall was to fit a small spoiler to
the back, and everyone said it hadn't taken Porsche too long to realise that
a spoiler was needed, and it was stupid that Audi hadn't done it from the
start. On many modern Porsches, the rear spoiler comes up electrically (it's
normally hidden) at a certain speed. It's been suggested that one way to
tell a porsche has had it's speedo disconnected (to freeze the odometer) is
if the rear spoiler isn't raised at motorway speeds.

Back to the Supra, this has an active front spoiler, which works just like
the Porsche rear spoiler. At 50mph, a lip comes down at the front to add
downforce. You can turn this on manually with a switch, but it will not come
or stay down below 30mph, so that you don't smash it on speed bumps and such
like.

On a Lamborghini Countach, thay are probably of some benefit. I think
that the original 'clean' Countach actually generated some positive lift
at the rear wheels, albeit at silly speeds. The spoiler was added to
help glue the rear end to the road. Of course, this might just be

On a Vauxhall Corsa, it's quite possibly purely decoration. At worse,
the downforce generated by the spoiler will cause the steering to go
light and the baseball capped twat will end up in the shrubbery. Or is
that 'at best'?

Halmyre

On the vast majority of cars, they're a fashion accessory.  Like front
fog lights, they mysteriously transform a run of the mill repmobile into
a 'sports car'.

Even when they do perform a useful function, they're unlikely to do so
at UK legal road speeds.  The VW Corrado had one that electrically
deployed at the appropriate speed but, as this was somewhere in excess
of the UK national speed limit, they lowered the speed for the UK model
so that owners could actually play with the gimmick.

My car before this one was the same make and model as the one before it,
with the same size and design of engine.  The only significant
differences were alloy wheels, distributorless ignition, air-con
(switched off most of the time) and the fact that they'd glued a spoiler
to the boot.  The first one would average 38mpg, but the second only
averaged 35 with the same driving style.   I always held the spoiler as
being most responsible for the drop.

Aerodynamics is a complex business and there are good designs and bad. It is
easy to bolt some pretty piece of plastic on and have it just produce worthless
drag forces.

A formula one car has spoilers for a reason, a Corsa 0.9 litre with a huge
spoiler, probably has it for a different reason.

It's easy to dismiss any add on as a piece of decoration, but even at ordinary
speeds a spoiler can be good aerodynamics.

As the name suggests to spoil the airflow over the car, a car acts much like an
aeroplane wing (a very bad one but the same physics and forces are involved) and
as speeds increase they generate lift as air follows a longer path over the top
of the car than the bottom it has to travel faster and this reduces the pressure
above the vehicle so the air under the car forces it up (however slightly) a
spoiler will break the flow of air from the car thus reducing the effect.
Spoilers should not be confused with wings as used in motor sport or the bolt on
fast and furious style ones, motor sport wings are designed to create downforce
by using the same principles as above but obviously the other way round so the
low pressure is below the wing so the higher pressure pushes down (there is no
suck or pull only push) and god knows what the fast and furious style ones are
supposed to do other than look tacky and part fools & there money.

Aftermarket ones are there to 'spoil' the appearance of the car.

very little or nothing on most cars

prettify ? see above

mostly

our C8 (the 807 and new Ulysse) has roof bars that can be slid together
to make what looks like a roof spoiler, looks nice but serves no purpose
other than to push fuel consumption up.

Mostly for 'decoration' - to make it look like the car is sufficiently fast
to warrant the need for downforce.

FWIW, the Escort Cosworth apparently had a need for a huge amount of
downforce at top speed to keep it on the road. Hence the huge spoiler that
can be seen from the moon in some cases. Which also takes 12mph off the top
speed, apparently.

Strangely though, come to think of it, the Lotus Carlton didn't have a huge
amount in the way of spoilers, did it?

Nick

99% of the time they're just there to help Police identify boy-racers and
other saddos.