Yes, it's true, improv teams have regularly scheduled practices. The best way to explain why we do this is to answer the most common questions about our practices.
"How can you practice making stuff up?"
Anyone can just make something up, but it takes a lot of practice to make up good stuff. As sad and pathetic as that sounds that's the easiest way to explain practicing improv. Not only do we practice standard improv rules over and over so they just become second nature to us, but we learn to make the more interesting decision in a scene. The best way to explain this is with an example. In real life we console people when they are upset or put themselves down. In an improv scene this is not interesting to watch so we basically do the opposite. In real life if your guy friend says, "I really screwed up that date" you tell them it's probably not as bad as they think. In an improv scene you can't do this because it's saying 'no' to what he said and it's not interesting. In improv you'll probably say, "Yeah you did. This restraining order already came in the mail."
"Is it really improv if you practice it?"
Yes. We aren't making up material for our next show. Anything we do in practice stays in practice. Those scenes are never brought up again in a show. Now if you're apart of a improv/sketch group and you have a great scene in practice then most likely someone will write it down so you can maybe perform it later for a sketch show. That's just part of the normal writing process. (Deep depression is also apart of the writing process.)
"Can you really teach someone to be funny?"
Short answer, no. Practicing improv isn't about learning to be funny. Most funny people have their own 'filter' that they view the world with. Practice is just a way to follow the rules and put them through your own 'filter'. There are tons of funny people who aren't good at improv. It takes a lot of practice to learn how to use your funny in scenes.
"Can't you just practice with another person and get better?"
If you're not an expert at improv you can only get so far just practicing with another person. It really helps to have an experienced person in the room who can teach you and help you become a better improvisor. That person is called a coach. You could also just practice by yourself and it's called improvsterbation.
"What's the difference between an improv class and an improv practice?"
In class you are learning the general rules of improv and sometimes the teacher will make you do exercises that specifically help you, but most of the time it's a general game or exercise. Improv practice is typically for your improv team. These are people that you will perform shows with so not only will you focus on stuff that makes you better at improv, but you'll work on stuff that will make you work better as a team. Oh, and class is more expensive.
Being a good improvisor means working muscles that you're not used to working in real life and training yourself to know how to react to different players and personalities. This requires a lot of practice. If you have any questions feel free to ask them in the comments section below.