A matter of preference and choosing the order you do things to get a different picture of things. Perhaps you only want to do conversations with one character for a while to get a conclusive idea of what they are like. Really, it's not about if it's needed, it's about preference. The only problem is that if you can't sufficiently clarify that both modes allow you to unlock and see everything, then it becomes a problem and I already confirmed that there are some lines that you will never see in vn, lines that are not important like asking the character if they need anything and they say no in their own way. It's not much.
Also if vn and sandbox differed, it would be unfair to those who only want to go through the game once with full completion.