Why is it so common to bunch email, todos, and calendars together? Is this based on some memory of a desk from the 1980s, where one might sit down to write a le

Why is it so common to bunch email, todos, and calendars together? Is this based on some memory of a desk from the 1980s, where one might sit down to write a letter to one's friend, check the calendar to see what date you should suggest meeting, and then mark on a sticky note that you need to get milk when you're at the grocery store?