@Rainbowdeer
Not everyone enjoys sea days, there are activities etc but some people like to get off the ship regularly.
Port days give you choices, you can go ashore early and come back for lunch, you can stay onboard and use the pools etc. when it’s quieter, have lunch and then go ashore, you can stay onboard and not go ashore.
Whilst the ships are very stable, you can’t predict the weather or if you will be affected by even a slight motion. As a first cruise having a port intensive itinerary means you know on most if not all port days you are docked and the ship will be stable. If you have someone in your party who finds they don’t like being at Sea or feels the motion plenty of sea days can help.
Personally I like sea days and do not always get off at each port but you are asking about a first cruise.