One example is analytics.

Lets say you want to build the next Google Analytics. You figure out you want to track IP Address, Browser & Device type. Later you, however, finds out you might want to track Browser Size as well. Since analytics databases can contain millions/billions of data, it isn't so easy to add an extra column to your table. It would simply take too long.

With a NOSQL solution you might just add it to the future rows, without having to change every row in the tracking table.

So in general "every" place where you need to crunch data and numbers for report