The Oracle of Omaha is adding to his already large stake in Apple Inc (AAPL.O).

Warren Buffett, 88, likes the technology giant because of its devoted customers, and has added to his stake in the company by “just a little” since his last regulatory filing, he said Thursday in an interview with CNBC. The Berkshire Hathaway Inc. (BRKa.N) chairman and chief executive officer said his firm has also bought back some of its own stock recently.

“They’ve got to keep having the product that this huge clientele regards as indispensable,” Buffett said. For customers, “the iPhone is enormously underpriced” compared with the utility it offers.

Berkshire has been piling more money into Apple, increasing that stake to 252 million shares as of June 30. The investment is worth more than US$50 billion and makes Berkshire the third-biggest shareholder in Apple, according to data compiled by Bloomberg.

Buffett’s company also boosted bets in Goldman Sachs Group Inc. (GS.N), Delta Air Lines Inc. (DAL.N) and Southwest Airlines Co. (LUV.N) in the second quarter.

Buffett has expanded his company into a conglomerate with a US$520 billion market cap and footholds in the railroad business, insurance industry and energy sector. With the help of deputies Todd Combs and Ted Weschler, the billionaire investor also oversees a US$180 billion stock portfolio that includes stakes in Wells Fargo & Co.

Buffett was in New York Thursday to dine with the winner of his annual lunch auction, which benefits San Francisco-based charity Glide. The winner paid US$3.3 million for the opportunity to bring guests to eat with Buffett at Smith & Wollensky.