Answer by taking a quote from the following article:

Norman Percevel Rockwell (February 3, 1894 - November 8, 1978) was a 20th-century American author, painter and illustrator. His works have a broad popular appeal in the United States for their reflection of American culture. Rockwell is most famous for the cover illustrations of everyday life he created for The Saturday Evening Post magazine over nearly five decades. Among the best-known of Rockwell's works are the Willie Gillis series, Rosie the Riveter, The Problem

Rockwell's family moved to New Rochelle, New York, when Norman was 21 years old. They shared a studio with the cartoonist Clyde Forsythe, who worked for The Saturday Evening Post. With Forsythe's help, Rockwell submitted his first successful cover painting to the Post in 1916, Mother's Day Off (published on May 20). He followed that success with Circus Barker and Strongman (published on June 3), Gramps at the Plate (August 5), Redhead Loves Hatty Perkins (September 16), People in a Theatre Balcony (October 14), and Man Playing Santa (December 9). Rockwell was published eight times on the Post cover within the first year. Ultimately, Rockwell published 323 original covers for The Saturday Evening Post over 47 years. His Sharp Harmony appeared on the cover of the issue dated September 26, 1936; it depicts a barber and three clients, enjoying an a cappella song. The image was adopted by SPEBSQSA in its promotion of the art.  Rockwell's success on the cover of the Post led to covers for other magazines of the day, most notably the Literary Digest, the Country Gentleman, Leslie's Weekly, Judge, Peoples Popular Monthly and Life magazine.  When Rockwell's tenure began with The Saturday Evening Post in 1916, he left his salaried position at Boys' Life, but continued to include scouts in Post cover images and the monthly magazine of the American Red Cross. He resumed work with the Boy Scouts of America in 1926 with production of his first of fifty-one original illustrations for the official Boy Scouts of America annual calendar, which still may be seen in the Norman Rockwell Art Gallery at the National Scouting Museum in the city of Irving near Dallas, Texas.  During World War I, he tried to enlist into the U.S. Navy but was refused entry because, at 140 pounds (64 kg), he was eight pounds underweight for someone 6 feet (1.8 m) tall. To compensate, he spent one night gorging himself on bananas, liquids and doughnuts, and weighed enough to enlist the next day. He was given the role of a military artist, however, and did not see any action during his tour of duty.

what type of art did it promote?
it depicts a barber and three clients, enjoying an a cappella song.