Pollyb,
welcome to Cruise-Chat.com.
What you have probably seen are so-called "roll-calls", where users post their cruise dates so they can get in touch with other users that might be booked on the same cruise. Many members have met for a drink or chat onboard and it's always nice to be able to know the people behind the user names. But of course it's a very subjective matter whether one wants to post his / her cruise dates or not.