Nah, it makes sense. You can write everything with just hiragana if you want to, in theory.
Katakana denote words from different languages, which I found really helpful when learning the language. It’s probably easier for fluent people too. There are a lot of these words.
Kanji are a lot more compact and make text a lot more readable. Japanese does not use any whitespaces so it can be tricky to separate words when using only hiragana. Instead you mostly have some kanji separated by hiragana. Some Kanji only denote a single hiragana, but usually they represent a group of them therefore saving on space too. Like other languages they have words with multiple meanings, but they have different kanji, further improving readability.
Take this with a small grain of salt, I’m by no means fluent myself, but I’ve been learning for quite some time.