They are used as a visual resource in language teaching.
Example
A teacher can use flashcards showing different faces in a lesson on describing how people look.
In the classroom
In the above class, learners can describe a face on one of the flashcards, and the other students can guess which one it is. They then follow this with an imaginary biography for the face they like most. Apart from being used for new vocabulary items, flashcards can be exploited as a prompt for extended skills work including discussion and creative writing.
Further links:
https://www.teachingenglish.org.uk/article/flashcard-maker
https://www.teachingenglish.org.uk/article/flashcards
https://www.teachingenglish.org.uk/article/kids-vocabulary