The Viking I and Viking 2 missions were designed to both orbit Mars and land and make exploratory observations on the planet's surface. At this stage in the history of the exploration of Mars, scientists had no idea what to expect nor what they might discover upon landing on the surface.
The landers confirmed the Mariner missions' bleak findings about the "lunar-esc" Martian landscape, and performed soil studies similar to those performed by Mars Pathfinder (MPF). The soils examined by the Viking missions were found to be similar, but unlike those explored by MPF!
Among other things, measurements from the Viking orbiters
helped establish the enormous size of Olympus Mons, which was first imaged by Mariner 9. Viking made more complete measurements of the shape of Mars,
measurements which help scientists understand the interior of the planet and the existence of the bulge called Tharsis Ridge.