The problem with beta software is that it is unfinished and may be unstable, which is why installing it onto an external drive rather than your main Mac hard drive might be a sensible option. By installing the new macOS 12.0 Monterey beta on an external drive, you can connect it and choose it as your main startup disk whenever you want, with no harm to your main Mac HD. The only downside is that it is quite a time-consuming process, and you will need to install an official macOS release (such as Big Sur) on your external drive first. But once done, you’ll be able to explore the future of macOS now, without affecting your Mac’s HD.