not necessarily samefagging, but it lets you take two approaches to a topic.
for example if i think you're an idiot, someone might want to both troll you and
have a reasonable argument to try and change your mind. add IDs and suddenly people have to pick - and they're never going to pick the high-effort reasoned argument over low effort trolling.
>Wouldn't you try to make bit more of an effort before posting anything if you know that subsequent posters will see how your posting progresses?
no. perversely it's easier to never make an effort because if you make a mistake it will undermine your entire point for the entire thread. that might be okay if we had threads that last 5 hours, but we have threads that last for weeks.
flags are a far superior identity forming system to IDs, particularly because they're totally optional. not unique like tripcodes, they introduce an element of genuine ambiguity to a long-term identity while still letting people guess if they wish. (plus obviously they let people signal their ideology without saying "as a posadist..." awkwardly in every post.)