The short answer is for comfort, it is not nice using a caravan that is not level, sometimes the fridge will not work, or the water will not drain away efficiently. It depends on the caravanners preference.
To level a single axle across the axle there are a number of devices on the market to help. From leveling blocks to air devices that use the car's exhaust to inflate.
I would recommend leveling blocks or the cheeper alternative, blocks of wood. Pull up on the desired pitch in roughly the position you require. Then have a look at the front of the caravan to gauge how un-level the caravan is and which side is lower. The place the leveling block/ piece of wood, behind the lower sides wheel and reverse the caravan back unlit the caravan has been lifted up sufficiently for it to be level. I recommend reversing the caravan onto the block as the brakes on the caravan work quicker when going forward due to the lead and trail brake shoes, apply the brake and un-hitch the caravan.
For a twin axle caravan the choices are a bit more limited, I would use varying different size of wood depending on the hight required. With a little practice you will be able to judge the hight required and correspond it with amount of blocks required.
I always carry some thick bits and a number of thin bits for final adjustment. With a twin axle I would decide weather to pull or push the van onto the wood depending on the tow car, the slope of the pitch and the surface of the pitch for grip. Obviously a twin axle caravan is a bit heavier than a single and you will also be pushing back against the brakes, so pitch surface is important to avoid the car's wheels spinning. (Site owners are nortously protective over their grass)
Put the caravan in the position you want it to be in after leveling, gauge the amount of wood required level the caravan and place them next to the wheels. Pull the caravan either back or forwards depending on the situation, then slide the wood into the position left by the wheels. This insures that the wood will have the correct spacing. Push/pull the caravan up onto the blocks and stop when the wheels are located Central on the blocks. Sometimes if the step up onto the blocks is too great, a smaller block of wood is required to act as a small step to stop the wheels pushing the main blocks away.
Once the cavan is level across the axle and has been un-hitched from the vehicle you can level the caravan from front to back. this is a simple case of winding the jockey wheel, situated at the front of the caravan, until the caravan is level. A spirit level can be used just inside the door to insure you get the level correct. After a while you will get used to knowing what is level and what is not just by feel. A bit of a tip thought, if the caravan front has to be raised to level, a trick of the eye means that it has too look slightly too high to be level.
Once the caravan is level the corner steadies can be lowered using the corner steady winder provided with the caravan, or using a drill with an adapter that can be purchased from an accessory shop.
Lower the REAR ones first until they touch the ground, some blocks may be needed to spread their weight on soft grass, then put weight on them by giving them and extra three turns or so. The caravan MUST NOT be leveled using the corner steadies, they are not designed to take the weight of the caravan.
Then once the rear steadies are down, lower the front ones in the same way.