I heard today that the Chazzon Ish says that the reason Parshas Parah is m'doraisa (See here) is because of the pasuk:
והזהרתם את בני ישראל מטמאותם
"And you shall separate the children of Israel from their uncleanness"
That through the reading of Parshas Parah b'tzibbur Klal Yisrael will know how to stay away from tumah. The fact that reading it b'tzibbur will be the best way to spread the news (a car with a microphone can't be sent around.)
This person had claimed to have seen this inside from the Chazzon Ish (in one of his seforim or perhaps brought down in his name.) Does anyone have a source for this?