I *need* multiple calendars
I need a work calendar, so my boss and cow orkers can arrange meetings and I can mark myself as unavailable as needed.
I need a personal calendar so I can track birthdays, personal trips, events and similar. I do not want my boss or coworkers to know my brother's birthday or that I'm visiting the STI clinic on Saturday.
I need a calendar for each of my organised hobbies, like the cricket, five-a-side match, synchronised swimming or amdram rehearsals so everyone involved knows to turn up - or that it's cancelled/postponed because not enough people can make it.
If you are working for multiple clients, you'll probably need even more.