In addition to the other responses here, we can consider Chaucer's The Canterbury Tales important on a variety of levels. Since this is really a question of value, how we evaluate the importance of a text varies.
From a historical perspective, this text is an important marker in the evolution of the English literary language. It is considered the first highly important work written in the vernacular, and it shows us what people in London sounded like circa 1390. The language was polyglossic, merging Anglo, Latin, and French in a delightful hodgepodge that highlights our literary ancestors. Because it was written, we can see the phonetic sound of many of Chaucer's words as well.
From a historical perspective, we also can find allusions to events that indicate cultural and political values. The Peasants Revolt and the emergence of mechanical clock towers are just two such mentions that allow us to see what fourteenth-century readers would be expected to recognize. While satirical, the range of character descriptions also paints a lively image of the diverse types of people whose qualities would be sufficiently recognizable to serve as objects of broad satire.
From a more purely literary perspective, Chaucer's tales are incredibly sophisticated and together form a collection of the many ways of telling stories in the fourteenth century. This catalog of voices and narratives can serve as a benchmark of what was popular in Chaucer's time and how he adapted stories from Boccaccio and others for his English readers. Furthermore, the psychological sophistication he imparts in characters like the Knight or the Wife is remarkable. While he decenters his own voice somewhat, Chaucer provides a sophisticated and highly ironic narrator for a battle-scarred man or a marriage-scarred woman. These resonate out of the text into subsequent generations, creating an oddly archetypal and psychological portrait.
Philosophically, Chaucer's tales engage in Platonic, Aristotelian, Augustinian, and Boethian thought, illustrating how the erudite philosophical debate about fate and chance plays out in quotidian life. The raging question of linguistic drift is also a valuable area of study in Chaucer, as is the difficulty of understanding the abstractions of the world when the words seem to mean too many things ("worthy" in General Prologue or "noble" in Wife's Tale, for instance). Chaucer's love of polysemy and puns further invites study of how we know what we know when language is so slippery.
Admittedly, Middle English can be challenging for a modern reader, though less so if read aloud. Very good translations have updated Chaucer's text for those who wish to read the text more readily. A novice to Middle English texts is well rewarded not only by the historical and literary sophistication of Chaucer's frame narrative, but also by the aesthetic pleasure it provides. These tales are really fun and funny individually and in the messy structure in which they seem to relate to each other.