Ignoring a topic does not make sense. There are hundreds of topic created everyday. None of it should bother you unless your notification for a specific topic is enabled.
It's got nothing to do with notifications. It's about what you see when you open a specific sub-forum. For example, look at Meta right now.
A few threads in Meta that I regularly see at the top include:
Merit & new rank requirementsReport plagiarism (copy/paste) here. Mods: please give temp or permban as neededWall of fame / shame. Shit posts so bad that they are actually funny Save your nice merit records here - LAST UPDATE: 06/03/2023Let's imagine that I want to be active in Meta but I don't want to see the above threads or some other ones I don't think are interesting to me personally. However, it makes no sense to me to ignore the users who created those discussions because I have nothing against them and I might enjoy there other discussions. Putting Meta on ignore is also out of the question. In those cases, it helps to put only certain threads on ignore, so as to not have to look at them every time you open a particular board. It has nothing to do with your watchlist or post notifications.
If I may add another possible benefit of what "ignore a topic" may offer, sometimes we want to leave a topic we're previously engaged on, either because the topic no longer interests us or it has turned into a meaningless discussion filled with spam posts. I am not sure there is currently a way to "remove" these threads from our "show new replies" page. The ignore a topic button would be a nice thing if it means it will remove those threads from my new replies page.