Mini whiteboards are an invaluable resource, and it is a great investment to have a full set in every classroom. It's even better when students carry their own whiteboard pens, as this makes it far easier for the practice to become habitual.
They are useful in several ways.
Whole-class formative assessment
The most powerful use of mini boards is assessing understanding. Picture this: your 5 minutes of whole-class questioning. You pose a question, pause, pounce on a student to answer. That’s 1/32 of the class's understanding you just assessed. Bounce it on to a couple of people: "Can you expand on X’s answer?", "Do you agree with Y?" - now you’re up to 3/32 of the class.
When students have mini boards, in the same amount of time you can elicit a response from every single person in the room. That means every learner engaging with the topic and showing you, through the validity of their responses, how much they understand.
Showing understanding during individual work
Wandering around the room, it is far too easy to get drawn towards helping particular students ("I’m stuck") without really getting a feel for the quality of what students are writing in their books. Does everyone understand, or are they all getting thrown by common errors and misconceptions? Some students even go out of their way to cover up work they're not sure about ("I haven’t finished yet!").
Enter mini whiteboards. When students complete the same task on mini boards, they collaborate better by looking at what each other are writing and it’s easy for the teacher to see too. They're less afraid of someone pointing out a mistake because it’s quick to make corrections. Perhaps students can start with Question 1 on whiteboards and then carry on with the rest in their books; then misconceptions can be instantly spotted before they are reinforced through flawed practice.
Quite a lot of students don't like putting pen to paper. In Maths, there is usually a single right answer and, therefore, a high risk of getting it wrong. Similarly, when writing paragraphs in English and other subjects, students sometimes lack self-belief in their own ideas and struggle to make a start. This can result in some learners taking a long time to write anything down, and perhaps even giving up altogether. The same students tend to be much more willing to start on mini boards because it's easy to wipe out mistakes if anything goes wrong. Once they've experimented and experienced a cycle of success (perhaps after some initial pointers), they will readily proceed on paper.