I think this is the key. At an accepted students day your kid can look around and see, and get to know a little bit, all the other kids who are very interested in the school and see if they feel like their kinda people. At some larger schools this may not matter, but at some LACs it could be the make or break. Depends on your kid too. You might have the kind of student who can get along with everyone or you might have the kind of kid who needs to find their tribe to fit in.
Is it a must to go to an admitted students day? Well there are plenty of kids from last year and year before who didn’t get to do many visits at all and are still happy with their choices, so no, not a must, but if you think if your kid is the kind who needs to be able to see other members of the prospective first year class in person then I think it could be incredibly helpful.