Why are full width and half width characters still a thing?

I apologise for a geeky post. I wasted hours trying to enter a postal address onto a Japanese website this week. I finally took the time to understand why I should be required to know how many bytes are being used to store it. Having read the Wikipedia page on the topic, I jumped to some conclusions about the real reasons behind how we find ourselves in this position in 2025. Can anyone help me find the silver lining to this cloud, i.e. why this is a good thing, or help me believe that in the future it will all be better?