Multiple reasons (this is not a comprehensive list):
You get placed into the queue randomly from the waiting room. Multiple queues gives you statistically a better chance at a better spot in line.
Queues can fail. If there's any glitch, you're out of luck if you only had one queue.
Generosity. When people get their tickets, they used to give away their queue space to someone else. I actually saved several people during Touch of Disney ticket sales back in 2021 because of a glitch that knocked out several people who were then told they had to just get back in line. I gave them my extra queues.