Hi BHSE, and a belated welcome to cruise-chat.com!
As others have mentioned, some passengers remain dressed up all evening while others change into more casual attire after dinner. Different cruise lines may have different policies, though. For example, Celebrity suggests that passengers remain in the stated evening's attire for the entire evening in most of the public areas. Royal Caribbean indicates that the dress suggestion is for the dining room only. Of course, on Royal Caribbean, you are free to choose which way you want to go.
As for us, I usually stay in my formal attire after dinner as it means less to pack. I have on many occasions, however, slipped on a pair of more comfortable shoes (sometimes even tennis shoes!).