No, it isn't required that you cast a circle for every spell. I often don't if I am doing something simple.
But, that having been said, there are two very good reasons why many cast a circle while doing rituals and spellwork.
1. The circle prevents any negative energies or malignant entities from disturbing you while you are doing your magical work.
2. The circle contains the magical energy you are raising until you are ready to release that energy to its intended goal.
