So, after I explained lots of times I do NOT cast circles or otherwise create sacred space, what ARE they great for, and when DO I do it?
In olde-tyme magick, circles were used to keep the nasties out. Many of the nasties in this case were what the magickian was invoking. They were a safe space to work in while the mage called up something to bend to her/his will. If them nasties ever got unionized, it would be REALLY ugly.
Most modern magick-users don't do that type of working today.
Now circles and other forms of sacred space are primarily (but not exclusively) rather a portal to the Otherworlds. In the case of a circle, you build your own universe with the Elements and often a God or Goddesses or both. In the Muglit ritual (the one we used in Mugwort Grove of ADF), we created our own universe with a Triad (usually Earth/Sea/Sky or Nature Spirits/Ancestors/Gods, and added The Well, The Fire, and the World Tree) then opened a gate between the worlds.
The sacred space might be called "between the worlds", it might be the creative center, whatever. You leave the created and go where all things are possible, all things are mutable. To the hidden reality behind our consentual reality. One step .... not beyond, but one step closer to Truth.
A cast circle will 'mute' or at least turn down the distractions of the mundane, and thus help a person move energy, do magic, feel energy, hear the God/dess/es, more clearly.
But a fully cast circle is time intensive, and far more 'formal' than the workings I usually do. Most magic is closer to Tweeting on Twitter than filling out a government RFP (request for proposal).
There are places for both, no question about it. But not everything requires doing a RFP, and not everything can be handed in 140 characters or less.