The reason I need to put this on multiple calendars is because I share my various calendars with different people. I am a consultant and I keep calendars for various clients. I give them access to see when I am free so when they want to schedule meetings they can see if I am booked or not. But I don't want them to see the details of my appointments, just that my time is booked. If I give them access to my main calendar with everything on it, they can see everything. If I just give them access to my calendar for them, they don't see I am booked elsewhere! Seems nuts to me that in this day and age we can't simply check which calendars we want an appointment to show up as "busy".
There is one obvious need - Those of us required to use exchange calendars for work cannot use the "time to leave" reminder on our phones without also entering the appointment in the default ios/icloud calendar since that function doesn't work with exchange calendars.
It happens to me all the time. On my iPhone, I want to see my work calendar and my home calendar. My home calendar is shared with my wife. She doesn't want to see my work calendar. Sometimes I have an even at work that I have to ensure my wife knows about so that she doesn't schedule something on the home calendar that will conflict. I don't want to take it off my WORK calendar, but I want to share it (and only it) on my Home calendar.