It might sound like a broad question, so maybe their actions between 1980 to then? What was the reasoning behind this?
The Nobel Committee press release from the award is suitably vague, but I know that at the time of the award UN troops were playing a significant role in maintaining post-conflict ceasefires in a range of theatres around the world including Afghanistan and in the Middle East. There were troops stationed in the Golan Heights and the UNIFIL mission in Lebanon in the wake of the 1982 war.
The explanation of the award in the [UN Chronicle] (https://www.questia.com/magazine/1G1-6944990/1988-nobel-peace-prize-awarded-to-un-peace-keeping) (the abstract is free but the full article is behind a paywall) from December 1988 mentions these deployments, amongst others, and that they:
"represent the manifest will of the community of nations to achieve peace through negotiations, and the forces have by their presence made a decisive contribution towards the initiation of actual peace negotiations".
So I think on the face of things it is fair to say that the prize was an acknowledgement of the increasingly comprehensive role played by international peacekeeping forces by the 1980s.
Part of the rationale might be a recognition of the way that peacekeepers were more tied into diplomatic efforts and peace plans by the 1980s, rather than simply standing as a physical bulwark against the resumption of conflict in a given locale.
A good example might be UNEF I in the Sinai in the period between the Suez crisis in 1956 and the Six Day War in 1967. UN forces were certainly instrumental in maintaining peace between Egypt and Israel through this period, but the peacekeeping mission itself was not particularly closely integrated into a political process to resolve the root tensions between the two states. When Egypt withdrew its invitation to UN forces, the countries were at war within days.
You can contrast an early example like UNEF I with a mission like UNIFIL in Lebanon which, although heavily restricted in its ability to carry out the full extent of its mandated mission due to hostilities through the 1980s and 1990s has maintained a role in the re-establishment of Lebanese political authority in conflict-affected zones, and expanded that role as the situation has allowed.
Thus a UN mission that has been active for nearly 30 years has grown from simply maintaining a peace, through to being a part of an internationally-monitored settlement, and taking up a wide-ranging role involving not just combat operations but including rebuilding social institutions, providing educational and medical services, and engaging in programmes to improve safety and mortality in a post-conflict environement (landmine clearance, etc.)
As to whether there was a particular micro-political or partisan motive behind the 1982 award is something I can't speak to, but maybe someone with more knowledge of the operations of the Nobel Committee could shed some light.