Sight radius determines effective use of the unit:
1.If sight radius is smaller than maximum range, unit needs spotters to fire at range.
2.If sight radius and range are both small, the unit becomes vulnerable to mellee units.
3.Larger radius aids scouting/exploration, and in general makes the map view less dark: if a play views the unit surrounded by darkness from all sides it creates uncertainty and requires memorizing past terrain, adding to stress.
4.In maps with no fog-of-war, speed of units is the most important factor: speedy units with smaller sight radius can aid discovering the map better than slow but high sight-radius units.
5.Some RTSs allow the entire map to be viewed by default or by researching some tech/upgrade(e.g. sattelite/radar/bird spies/etc). While it eliminates the sight radius advantages, such map lack surprises and behave as complete information games (ala chess/go).
6.Buildings usually have much higher sight radius to compensate for lack of mobility.