Stop & Shop workers used a strike that effectively shut down 246 stores in three states to preserve health-care and pension benefits, contracts ratified by five United Food and Commercial Workers local unions show.

The company had consistently offered wage increases to all full- and part-time employees during negotiations. But it sought concessions on health-care coverage, pensions, and premium pay on Sundays and holidays.

Workers went on strike to preserve their benefits, said Charles Allen, a retail analyst for Bloomberg...