Are end panels necessary? This depends on the door style that you have opted for as most of the carcases match the doors almost identically.
Some people do buy an end panel if they know the unit isn't going to butt up to a wall. An end panel can also be used to fill a small gap of about 20mm but a cheaper alternative would be to use a filler or a piece of plinth.
In a high gloss kitchen it will be beneficial to purchase matching end panels for these as the carcases have a matt finish and would look dull against a gloss door.
You may also consider buying end panels for painted kitchens as they will be colour matched.
In some ranges there are tongue & groove end panels available which will add an extra decorative feature to your kitchen.