Some shows are so popular that they become cultural touchstones, like shared memories for the population. The finale of M*A*S*H had nearly 106 million viewers. The only show to beat that was the 2010 Super Bowl. Similarly, events like the assassination of President John F. Kennedy and the first moon landing are remembered as events on television because that's how everyone experienced them.
People imitate the fashion and style they see on television. Jennifer Aniston's "Rachel" haircut on "Friends" spawned thousands of imitations. In the 1980s, "Dallas" and "Dynasty" brought back big-shoulder dresses and lots of jewelry, gold and glitz.
TV has helped in the cause of race relations--from back in the 1970s, when Sammy Davis Jr. kissed Archie Bunker on "All in the Family," up to Bill Cosby's groundbreaking "The Cosby Show," which showed a black family that was upper-middle class and rarely used race as a backdrop--and was hugely popular.
Gender also became an issue, then a non-issue, with "Will & Grace" popularizing gay characters on television, helping to portray a segment of the population that had been marginalized in the past.
Some say that reality TV gives a distorted view of the world to viewers, but there are also benefits to this kind of television. "American Idol" brought pop singing back in a big way; "Dancing with the Stars" sparked a huge interest in ballroom dancing; and "The Amazing Race" teaches viewers about parts of the world they're rarely likely to visit.
Some say that watching TV causes a sedentary lifestyle because people don't exercise while they view their shows. But with programs like "Dr. Oz" regularly preaching healthy living and "Ruby" showing the perils of obesity, even that argument is no longer valid.