I like to keep separate calendars because they can be color coded, letting me tell at a glance that I have plans and what part of my life will be affected by any changes. I keep one each for my bookkeeping business, my genealogy commitments (GenSoc, WikiTree, education), and my personal stuff (drs, hairdresser, etc). The rest are calendars I share with others for various reasons.
Different calendars also lets me set default behaviours like when and how to remind me. I like my genealogy things to remind me once at thirty minutes and once at 5 minutes with a vibration of my phone. My business stuff reminds me five time, once every quarter hour and at five minutes also with vibration on my phone but a different pattern. The business also alerts on my tablet but the personal and genealogy don't. My husband has his set to send him an agenda by email the day before.
My life is under control thanks to my calendars. Without them, I focus on what I want to do rather than what I need to do.