Many witches don't cast circles. Circles are not apart of every culture.
Circles are generally only used when you feel you need extra protection or no spiritual distractions when casting. Circles prevent outside spiritual interference and create a sacred space to work in cleared of all but the energies you want there. If it's a simple spell it isn't necessary. But if it's for a serious matter, better safe than sorry.
It's like an umbrella. Some people carry umbrellas with them at all times, "just in case". Some people bring it with them when the skies are cloudy or just when its raining. And some don't care if they get wet or not. And some love the rain! Lol
And no, you don't have to recast a circle when performing more than one spell. But I wouldn't recommend performing multiple spells at once regardless unless they tie into each other and work together. If you're insistent on casting more than one spell for different things, I would recommend cleansing the sacred space inbetween castings so the different energies don't mix and interfere with each other. Don't disperse your previous spell though, let the energy of your work leave the circle and go on its way to produce whatever results you desired from them, then cleanse the space.
I see circles like semipermeable membranes controlled by our mind. They let in and out only what we want them to. That's why many witches use circles, it gives them great control over their environment when they practise.