These Five Dialogues cover the most famous (or infamous) portion of Socrates life--his trial and subsequent execution. In this they are historically invaluable.
Plato lays out each dialogue with great artistic prowess (and the translators, for their part, keep everything smooth and pleasant). Society has fully internalized this art and anyone unfamiliar with it is at a disadvantage when considering anything subsequent found in the Western tradition. In this they are culturally invaluable.
One finds also in these dialogues the very basics of Platonic thought--most notably the theory of Forms. There are, of course, many other concepts introduced, from politics to metaphysics. In this the dialogues are philosophically invaluable.
If I haven't yet convinced you to pick up a copy of Five Dialogues I don't know what will, but perhaps you would be interested to know that the book contains good (but short) introductions to each dialogue, informative (although rare) footnotes, and an extensive (if outdated) suggested reading list? I hope that did the trick, because this set of dialogues is, well, invaluable.