why do I need .zshrc?

On my MacBook Air, I needed a .zshrc file to convince the machine to use my selected terminal prompt (e.g. export PS1="prompt"). On my iMac, I didn't need that. The terminal prompt was somehow already correct. But I have .bashrc files on both machines, and they do a fine job with maintaining system aliases. Both machines are on Sequoia 15.3.2. I am told that Macs REALLY want to use .zshrc. Why? Do I just move all my .bashrc commands over to .zshrc and kill off .bashrc? Are my .bashrc files eventually just going to stop working?