Why does this tool in the language need to exist? The answer cannot be because Kanji make things easier to read, else you wouldn't need tools to help you read Kanji you at times otherwise wouldn't be able to.
If you come across a word you don't know as either a native speaker of English or Korean, you can at least sound it out, which ups the probability you can connect it with a word you've heard before, otherwise since you know how to type it out it's trivial to look it up in a dictionary. If you come across a word you don't know in Japanese as a native speaker and there's no furigana it's a guessing game. The meaning is slightly more obvious to you, so you might be able to guess, but if you can't guess and you care to know and the word is in print then looking it up becomes a bit more of a pain.
Korean didn't completely lose the mutual intelligibility aspect entirely since the underlying pronunciation of the words still remains and can be used to correctly guess the word in a lot of cases. Like 시간 and 時間 as an example, but there's many, many words I've been able to guess in Korean based off knowing Japanese. I was able to score 50% on TOPIK II reading exam after only having studied Korean for 4 months in large part because of this.
