They are twassocks
Fell for the hype
Think it looks cool
Cos their mate has the same
Read too many fairy stories as a kid
They like the noise.
Loads of reasons...what you won't find is anyone who will admit to the mods being a failure.
Very similar to many "go faster" gizmos that you can spend your money on.
If you are tuning the engine so that it needs to breathe better, induction kits make sense, otherwise, swapping your OEM air filter for a clean new one likely has more benefits
Modern cars have far less ventilation under the bonnet then older models so the bonnet space is hotter.
Makers go to a lot of trouble to get air from outside...where it will be cooler.
Cooler air is denser and can hold more oxygen so the ECU bases a lot of fuel/air calculations and when to fire the spark on air temperature