The problem with opening multiple terminals is that you will eventually be slowed down in your workflow. Say you want to switch to a particular terminal, and you have more than 10 terminals running. You either have to press CTRL+Tab a few times (5 times on average for 10 terminals), or switch using the mouse, both of which are slow.
Another problem is working remotely. If you connect to a remote server, and you need another terminal for that remote server, you must establish another separate connection. With a terminal multiplexer, you do not need that.
Another problem is working remotely. If you connect to a remote server, and you need another terminal for that remote server, you must establish another separate connection. With a terminal multiplexer, you do not need that.