Presumably because they want a higher degree of privacy / urgency when talking about their plans or the intimate details of their lives, sharing selfies or pics of the kids, fighting, flirting / sexting, and when sharing nudes, but OP doesn’t care about that higher degree of privacy for their friends who they don’t discuss those things with.

And the reason to have a separate app for work safe conversations only with her makes sense since it allows conversations to be nsfw without any extra worry (e.g., will I accidentally open this later when not at work? What if I think you’re off work already but you had to stay late to have a conversation with your boss?) or hassle.

And only having one app for everything else could be because he doesn’t have nsfw conversations there and thus can safely check them at work or because he doesn’t have important conversations there and thus doesn’t need to check them while at work. Or both.