The swords that the Hobbits got from the Barrow-downs were apparently magical in some way: in "The Departure of Boromir", Aragorn says this of Merry and Pippin's blades:
Doubtless the Orcs despoiled them, but feared to keep the knives, knowing them for what they are: work of Westernesse, wound about with spells for the bane of Mordor.
There are several reasons to believe that the Barrow-blades were particularly harmful to the Nazgûl. A major piece of evidence is the effect of Merry's blade on the Witch-king during the Battle of the Pelennor Fields. It was enchanted with the power to harm the Witch-king of Angmar himself.
We know almost nothing about whether the Barrow-blades had any special effect on other evil creatures. In "Flotsam and Jetsam", Merry says that Uglúk (leader of the Uruk-hai band) took the swords but then "threw the things away as if they burned him."
(Details of question quoted from here.)
With that said, how did the Hobbits get their Barrow-blades back if the Uruks threw them away after Merry and Pippin were captured?