If you don't drive your car for a year the battery will go dead and may not be any good anymore especially if you don't have a battery tender on it or trickle charger to keep the battery charged.
Also if a car is not driven for a year and sits in one spot the tires can start to degrade and get flat spots and if on dirt the cars tires may even sink into ruts into the ground.
Moisture can also accumulate in the engine and engine components and lead to rust and corrosion and fuel or gasoline that is left in the tank can turn bad and turn into varnish and cause clogged fuel filters, clogged fuel lines, clogged fuel injectors and make it hard or difficult or impossible to start the vehicle.
In severe enough cases if the gasoline goes really bad in the cars gas tank you may have to drop the gas tank and and flush the old gasoline out of the tank and then clean the gasoline tank and put fresh gas in.
A vehicle that sits for longer than a month should have some gasoline in the tank so that you can start the car or vehicle up at least every few weeks.
This keeps the gasoline fresh through the system and also starting the vehicle up allows the battery to charge up and allows fluids like oil and coolant to flow through the system and also keeps the seals livened up.