As many have mentioned the real reason is to suppress votes by making the experience miserable.
The cover story for the rules is to prevent campaigns or other groups from “buying votes” by giving people in line food/water in exchange for a promise to vote for their candidate.
I was once on a Greyhound bus where a guy was loudly talking on his phone, but not on speaker. The issue was he was just repeatedly saying:
“Listen… Listen… Listen… Listen… Lemme tell you… Listen… Listen … Listen lemme… Lemme tell you … listen”
For half an hour.
Finally a hero yelled out: “Listen! Lemme tell you to shut the fuck up!”
He said “lemme call you back later” hung up and shrunk down on his seat as the whole bus clapped.