Altars, is it necessary for witches to have one? In short, yes. I feel it is. However how the altar is set up and the size and permanence of said altar is very personal.
Altars are sacred spaces, a place to work majick, celebrate sabbats, reflect, meditate and appreciate. It is where we are able to focus our intentions and grow as spiritual beings.
Typically a basic altar has candles, a cauldron, a vessel to hold water and a place to burn incense/sage (the cauldron can be used as such). Personally my altar can be set up and put away very easily and does not take much space, since my house is very small. Some people have portable altars that are the size of altoid tins. I have a portable one for those times I decide to work some majick at the ocean or in the forest. I have a mini altar set up on a small shelf I had decorated, that is up all year round and I decorate it according to the sabbats. My main altar (photo above) is strictly for ritual and big spell work.
I find that new pagans get overwhelmed with altars and how to set their sacred spaces up. The one thing I like to elaborate on is, use whatever you resonate with, the symbolism, items you may find in your travels. You don't have to spend a lot of money on altar goods, you can use what nature supplies, use a stick you found on the ground as an altar, get candles at your local dollar store, find chalices, cups, bowls and little herb/jewelry cabinets at your local savers or thrift stores. Be creative! The more of your energy you put into an object the better! If you look online there are so many different altar ideas out there, that it is quite fascinating to see. The main thing is, have fun with it, and enjoy building it!
What does your sacred space look like?