One of the Univ Houston lectures on Vikings explained that riddles and sagas were part of the entertainment during those long winter nights.
But this lecture discusses how riddles were used in Anglo Saxon times.
There are several sources of riddles from the Anglo-Saxon era. The largest is the Exeter Book, a tenth-century collection of Anglo-Saxon poetry. Orchard believes that the this work originally contained 101 riddles, but the manuscript has been damaged, leaving only 91 riddles. Other sources of Anglo-Saxon riddles include the Berne and Lorsch collections, and in the work Carmen de virginitate, by Aldhelm, Abbot of Malmesbury (c.639-709). Orchard also notes that Bede, Alcuin and Boniface all wrote riddles, and that these examples show that they were part of “a learned literate Latin tradition” in Anglo-Saxon England.preview link
Part of the reason riddles were so popular was that they had a lot of pedagogical value – they could be used in schools to get students interested in language.