Written texts were expensive during the era before the printing press, both in time and supplies. Do we know why the Icelandic monks spent a lot of time and resources to document the oral Norse tradition of the Sagas and pagan Norse mythology?
/u/Platypuskeeper and I answered at least the latter from a bit different point of view (while he put much emphasis on the cultural aspect, I concentrated on the political background of the intensified power struggles both in Scandinavia and in Iceland in the late 12th century) in this thread: Why did Snorri write the Prose Edda?
(Added): Note also that some Norwegian kings in the 13th centuries like Sverre Sigurdsson (r. 1177/1185-1202) and his grandson, Håkon Håkonsson directly commissioned the Icelanders to put their biographies down as a saga. I suppose they (the newly established Sverre-Birkebeiner [a political faction in Norway in the Civil War periord] dynasty), essentially as ex-usurpers in the dynastic struggles with dubious legitimacy, sought some historical sense of the dynastic continuity by adding their sagas to the series of royal biographies of Norway from legendary times, and it was one of the few areas of activity [cultural capital Cf. Warner 2008] that the Icelanders like Snorri Sturluson can actually (?) contribute to. Snorri also tried himself hard to establish the patronage from several Birkebeiner rulers, including King Sverre and Håkon.
As for the eager attitude of such rulers to the sagas of rulers in the past, the best representing episode is to be found in the Saga of Håkon Håkonsson, in the protagonist King Håkon's deathbed, I suppose:
'In the sickness he let Latin books be read to him at first. But then he thought it great trouble to think over what that [the Latin] meant. Then he let be read to him Norse books, night and day; First the Sagas of the saints; and when they were read out he let be read to him the tale of the kings from Halfdan the black, and so on of all the kings of Norway, one after the other......' (HsH, Chap. 397; Dasent trans. 1894: 366)
Additional References:
+++