Circles don't have to be used, some people use them for everything, others don't use them at all.
Circles can be used to call in deities, ancestors, elements, etc into your cirle to help with a spell, they can also be used to build up energy befoe you release it. Because of this there are some spells that simply don't need a circle, but some people enjoy the ritual of it.
For cleansing it is also personal preference, some people cleanse before every spell, while others may only cleanse when there has been negative energy in the space, it is all personal preference and what you feel confortable with.
If you want to cleanse and cast a circle everytime in order to feel comfortable, or don't like the ritual, then it is best to do that than follow what feels right for you, rather than what everyone else is doing.