Hiya and welcome to your first cruise...you will love it!
We have done both and much prefer flex/freedom dining.
1st, The main reason is I can eat when I want. Nothing worse if come off a tour and back in cabin you fall asleep and wake 20 mins before your sitting. One mad rush to get ready. There might be a show you want to see or have drinks with another couple you have met...then the choice is yours how you spend your evening.
2nd, Meeting the same people every evening can get a bit monotonous, especially if you have someone on the table who commands attention all the time!...meet so many more people by changing each night.
3rd, I feel like eating at the buffet tonight or another speciality restaurant. I can just go. Politeness is always to tell your table you won't be there the following night, but I didn't know then I fancied Buffet this evening!. Nothing worse than being on a table and you are waiting for a couple to turn up who didn't have the manners the night before to say they wouldn't be eating tonight with us.
Enjoy........Chris