They played a crucial role in weakening the forces of the Easterlings which were likely to join the forces of Sauron.
"The 'other two' came much earlier, at the same time probably as Glorfindel, when matters became very dangerous in the Second Age. Glorfindel was sent to aid Elrond and was (though not yet said) pre-eminent in the war in Eriador. But the other two Istari were sent for a different purpose. Morinehtar and Romestamo. Darkness-slayer and East-helper. Their task was to circumvent Sauron: to bring help to the few tribes of Men that had rebelled from Melkor-worship, to stir up rebellion and after his first fall to search out his hiding (in which they failed) and to cause dissension and disarray among the dark East. They must have had very great influence on the history of the Second Age and Third Age in weakening and disarraying the forces of the East who would both in the Second and Third Age otherwise have outnumbered the West."
History of Middle-earth - Chapter XII: The Peoples of Middle-earth
It is speculated that, without them, Gondor would have been overwhelmed by the number of Easterlings in the War of the Ring.
Further notes about them:
Of the Blue little was known in the West, and they had no names save Ithryn Luin 'the Blue Wizards;' for they passed into the East with Curunir, but they never returned, and whether they remained in the East, pursuing there the purposes for which they were sent; or perished; or as some hold were ensnared by Sauron and became his servants, is not now known.
Unfinished Tales - Part Four Chapter II: The Istari