Mostly, splitting the cost of a tour.
Six people sharing a private driver in a port is a completely different price from six people buying coach seats from the ship, and it is a better day out. That single fact is why roll calls exist and why they get organised months ahead.
After that: finding other families with kids the same age, working out who else is doing the same excursion, and the low-grade reassurance of recognising two faces at the first dinner.
You are not obliged to meet anybody. Plenty of people read their roll call purely for the port advice and never post. That is a completely normal way to use one.
What has a roll call actually got you that you would not have had otherwise? Worth hearing from people who have used one properly.
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