As a Japanese learner, katakana is a godsend. It’s like reading a scientific paper in English and having all the Latin in italics, as an indicator that “don’t worry this is a foreign word, you’re not an idiot for not recognizing it.” Especially because most katakana words are derived from English (or words you’d recognize as an English speaker) so it’s just a matter of saying it over and over until the pieces click into place. Example: オーストラリア = Oosutoraria = Oh-s-t-rah-ree-uh = Australia.
Also outside of picture books for young children, Japanese doesn’t use spaces and has way fewer sounds than most languages which results in a LOT of homonyms and similar words that all blends together (see other comment YouTube link). So having three writing systems in one really helps convey meaning and makes reading much faster.