It really depends on what kind of work is being performed and how it needs to get “pushed out”.
A circle performs a number of functions. It does effectively keep undesirable energy out, but it also keeps things in, which can work for or against you. It can be used to magnify and accelerate the Work, so that opening the circle releases the Work in a very expansive way. If working with others, it can help to develop the group mind.
Yes, it makes a difference. Wether or not it’s important, well, that kind of comes with practice.
For example, if I am doing work to release a belief that is no longer serving me (“I need cigarettes!”) I would probably NOT use a circle, as it would hold that belief in with me while I do the work. However, once the work is done and I am prepared to do healing work (“I will strengthen my lungs!”) I would. Make sense?
For Wiccan Ritual, OTOH, I almost always “cast circle” but it’s slightly different. This is part of the creation of the temple and is necessary for performing the ritual properly. In this case, it’s more about having a bouncer for the party.