Their physical presence makes it clear that the group is taking everyone’s limits seriously. Compare “Just say stop if you don’t like something” to “Everyone has a button. I care about everyone and want everyone to feel comfortable. Press the button if you’re uncomfortable.” The phyiscal aspect makes it extremely clear that the group has gone out of their way to make it a priority. It’s hard to speak up for yourself. It can be awkward. Knowing that the group took it seriously enough to get physical objects for it will help people understand they’re more welcome to stop things.
Pushing a button or tapping something can be a lot easier than saying something. A lot of people can get nonverbal when they’re uncomfortable.
I think that it serves two purposes: