Install if You Work in IT
Do you use Macs at work? If you are an IT manager, a power user or a person in charge of software and processes in a business you should install the macOS Catalina beta and test it out with your software and your specific business functions ahead of time.
Of course, you should do this with permission and in the right way so that you aren't going to bring down a production environment, but for many IT users this is something that needs to be done.
You may not want to install the first or second macOS 10.15 beta, but once it is more stable and developers start ot offer beta versions of their software this is a good idea.
You can use this time to test with specialized hardware and with custom apps and essential business software so that you can provide feedback to Apple and to the developers about what works and what doesn't. The more specialized your setup is, the more important this reason to install is.