I just went to look in my game because I didn't know it was a thing. I was playing a newish save and had one married sim travel to another married sims house and do an introduction and sure enough it automatically shows they have a spouse and who it is without asking. Neat.
Maybe it was added in later? Heck it could have been there all along for all I know.
Anyway if you are asking why they should have it like that it's another tool.
I have my sim ask for immersive gameplay purposes usually even if I know the (usually premade) sim is married.
The info might also be there for those simmers that want no romantic interactions with married folks... some feel very strongly about that. Or I do remember threads where simmers where saying all the sims their sim meets is married. So they can know by looking if the sim is available to be approached. Just another tool for those who want to use it.
Good deal.
