Let's say we're building a calendar app, like Google Calendar. The app lets you toggle between three different displays: month, week, and day.
Personally, I always want to see the "Week" view. It gives me everything I need to know about the current day, while also giving me a peek at what's coming up in the next couple of days.*
Thankfully, calendar apps know that users have strong preferences around this kind of thing, and the toggle is “sticky”. If I switch from “week” to “month” and refresh the page, the “month” view is the new default; it sticks.