In theory, when a company is promoted or added to an index, it benefits from being lumped in with other, more robust companies. By joining the Russell 1000, AMC will be part of an index where 92% of the entire market capitalization of all U.S.-listed stocks is concentrated. As a result, investor demand may rise, as well as trading volume, making the stock more liquid and desirable.
In the Russell 2000, these companies were among the largest and most dominant. Usually, they were among the top 10 holdings by market cap for much of the year.