Plus-Sizing or Up-Stepping are two terms given to the practice of increasing the diameter of your wheels whilst simultaneously reducing the profile of your tyres to keep the overall rolling radius the same. Plus sizing helps improve the handling of the car.
Benefits - Plus-Sizing will improve the handling of your car! - each step will reduce the proportion of flexible tyre 'sidewall' to rigid alloy. This will improve response, will help keep the tyre tread square to the road and will improve your car's 'feedback'. If done properly speedo and odometer accuracy will be retained and the car's sure to look better..
Disadvantages - In the majority of situations, tyre inches are lighter than wheel inches. Plus-sizing can make your overall wheel/tyre package heavier. Reducing the profile of your tyres will also reduce your car's damping deflection under compression [the ride quality will get worse] Other disadvantages can include you needing more expensive tyres, your brakes looking puny and people's grannies laughing and calling your car a pram.