Although about half of Shakespeare's plays had already appeared in individual publications, in a much cheaper and smaller format (known as quarto), the First Folio contains 18 plays that had never been printed before, some of them amongst the most famous in the canon.
Without the Folio, we would not have Macbeth or Julius Caesar, Antony and Cleopatra or Coriolanus; we would be missing Twelfth Night and As You Like It, Measure for Measure and The Comedy of Errors; we would be without The Winter's Tale and The Tempest.
We would never had heard of "Et tu, Brute", or "All the world's a stage", or "If music be the food of love, play on", or "O brave new world, that has such people in it!"
Some of the other plays, which had already been printed, appear in the Folio in versions significantly different from those in which they were first published.
The Folio text of Hamlet is missing 230 lines that are present in the previously printed versions, but has 70 lines which those versions lack.
King Lear has a series of cuts and rearrangements which strikingly alter the version printed fifteen years earlier.
Henry V, Romeo and Juliet, and The Taming of the Shrew all differ in various ways from the previously known editions.
These changes tell us that Shakespeare's text was never stable, but that the printed plays carry signs of adjustments that were made as soon they entered the theatre.
They even suggest that Shakespeare may have revised some of his plays, either to reshape them for publication, or to meet the changing demands of performance.