These were sometimes used to assist in the learning of touch typing. Clients would have their local repair person install these little black paper circles to cover the letters so they wouldn't be tempted to stare at the keyboard. Fairly rare, I've not seen them used on any of the 500+ machines I've serviced in the past 7 or so years. There were also dedicated blank caps designed to go over the keytops that were used, those are much more common than the blackout paper method.

Even with keyring pliers and the skill to use them, the blackout paper method is a lengthy one.

Full alphabet sentences, also called pangrams, were used to check the function of all letters on the typewriter keyboard. They were also used in typing lessons.