A circle does two things; it creates a clean, balanced space for magick and it also creates a containing barrier for the energies involved.
If you feel your magickal working is especially delicate or cannot risk being contaminated; banish. This form of circle creation will give you a space of perfectly balanced energy within which you can do your magickal workings. If you are going to be summoning or generating a large quantity of energy to focus into something (such as when you charge an item like a sigil, mojo bag, or talisman to wear) you should create a circle just so the summoned/generated energy doesn't dissipate before you can use it.
In smaller, simpler workings or workings that are more a form of worship may or may not require a circle depending on who you ask. Wiccan-inspired traditions are a little more giving with circle-free castings, as are traditions that are primarily religious in nature with magick as a bit of an afterthought (such as with Asatru). Hermetic traditions are much more likely to insist that you cast a circle every time. Heck, I've even been advised to also do the LBRP upon waking and just before bed in addition to doing it before ritual... Hermeticism can get a little banish-happy.